earthing system & protection in low voltage …

Shrivastava [Type the company name]

[Pick the date]

EARTHING SYSTEM & PROTECTION IN LOW VOLTAGE INSTALLATIONS VOL.1: BASICS OF LV EARTHING SYSTEM

CAMTECH/ EL/ 2020-21/ Earthing-Vol.1/ 1.0

March 2021

END USER: RAILWAY ELECTRICAL ENGINEERS

AND TECHNICIANS DEALING WITH LV

INSTALLATIONS

Maharajpura, GWALIOR - 474 005

ii

EARTHING SYSTEM &

PROTECTION IN LOW

VOLTAGE

INSTALLATIONS

VOL.1: BASICS OF LV EARTHING SYSTEM

iii

“We at RDSO, Lucknow are committed to maintain and update

transparent standards of services to develop safe, modern and cost

effective railway technology complying with statutory and regulatory

requirements, through excellence in research, designs and standards by

setting quality objectives, commitment to satisfy applicable

requirements and continual improvements of the quality management

system to cater to growing needs, demand and expectations of

passenger and freight traffic on the railways through periodic review

of quality management systems to achieve continual improvement and

customer appreciation. It is communicated and applied within the

organization and making it available to all the relevant interested

parties.”

QUALITY POLICY

FOREWORD

Use of electricity in day-to-day living is an integral part of everyone’s life. Since unintended

exposure to electricity is life threatening, provision of proper earthing is essential for any

electrical wiring and installation. Most of us treat earth electrode as solution of safety from

unintended exposure to electricity which is just not the case. There are also so many myths and

misconception about earthing and earthing systems. To ensure safety of life and apparatus

against earth faults, it is necessary to provide proper earth connection to equipotential bonding

along with other suitable protection devices to disconnect the electric supply within the

stipulated time. For understanding these concepts, the electrical engineers and technicians shall

be familiar to correct code and practices.

To break the myths about earthing and to ensure wide dissemination of knowledge on correct

practices about earthing in LT installations, this volume-1 on “Basics of Earthing System”

under the document “Earthing System and Protection in Low Voltage Installations” has been

prepared by CAMTECH, Gwalior.

This volume-1 covers common myths and facts along with detailed explanations. This also

contains relevant terms, circuits of earthing systems (like TN, TT, IT), effect of electric shock on

human body, IE/ CEAR rules/ regulations pertaining to earthing, etc.

I am sure that this booklet will be useful for electrical design and maintenance engineers and

technicians for updating their knowledge, improving the reliability and safety of electrical

installations as well as precious human lives. This will be also helpful in reducing the electrical

failures on account of earth faults.

Jitendra Singh

Principal Executive Director

PREFACE

The “code of practice for earthing of power supply installations for 25kV single phase traction

system” is covered in Appendix-III of ACTM Vol-II, Part-II. It is important to note that Low

voltage (LT) electrical power distribution system does not come under the purview of this code.

(As per para-1 of the appendix-III).

There are Indian Standards and regulations on earthing and earthing system which are neglected

and misinterpreted at many places during design and commissioning of system. There is no other

standard document for LT earthing on I.R..Railway Board therefore, advised CAMTECH,

Gwalior to study the subject in details and address the common myths about LT earthing in the

publication to disseminate the knowledge among the electrical engineers and technicians about

correct practices of earthing system.

The word “earth” and “earthing” is misunderstood as an electrode in soil and similarly there are

so many Myths and misconceptions related to earthing and earthing system which are deeply

incorporated as wrong practices in the regular working systems. The earth connection improves

service continuity and avoids damage to equipment and danger to human life. It also reduces the

risk of a person in the vicinity of earthed facilities being exposed to the danger of critical electric

shock. The object of earthing is to ensure safety of life and apparatus against earth faults

This volume-1 on “Basics of LV Earthing System” under the document “Earthing System

and Protection in Low Voltage Installations” has been prepared by CAMTECH, Gwalior with

the intention to spread awareness on myths and facts about earthing in LV installations, types of

System earthing, Equipotential bonding and other terms related to LV installations to all those

who are concerned with the design, installation, inspection and maintenance of electrical systems

and apparatus.

Technological up-gradation & learning is a continuous process. Please feel free to write to us for

any addition/ modification in this booklet. We shall highly appreciate your contribution in this

direction.

Himanshu Maheshwari

Dy. Director /Electrical

vi

ACKNOWLEDGEMENT

I would like to express my sincere gratitude to Shri Jitendra Singh, PED/CAMTECH for the

continuous support, guidance & motivation in making of this publication.

I would like to thank Shri Manish Gupta, ED/EEM/Railway Board for his continuous

encouragement, direction, support and guidance during in making of this publication.

My sincere thanks to Shri Vivek Dixit, CEGE/Central Railway for his continuous motivation,

guidance, suggestions, insightful comments, and hard questions which guided us to make this

publication more simpler for users to understand.

My sincere thanks to Shri S Gopa Kumar, Member in various Electrotechnical committees of

BIS for his technical guidance and support throughout the process of making this publication.

He has helped us in understanding the Indian and international standards, various concepts of

earthings, comments and clearing our doubts which we faced during the making of this

publications.

I thank my team members Shri B. C. Agrawal SSE/EL/CAMTECH & Smt. Sangeeta

Shrivastava JE/IT/CAMTECH for their continuous study, hardwork, sincere efforts &

dedication to make this publication.

Himanshu Maheshwari

Dy. Director /Electrical

DRAFT

iv

CONTENTS

Description Page No.

FOREWORD ................................................................................................................................. iv

PREFACE ....................................................................................................................................... v

ACKNOWLEDGEMENT ............................................................................................................. vi

CONTENTS ................................................................................................................................... iv

LIST OF FIGURES ....................................................................................................................... ix

LIST OF TABLES .......................................................................................................................... x

ISSUE OF CORRECTION SLIP................................................................................................... xi

DISCLAIMER .............................................................................................................................. xii

OBJECTIVE OF PUBLICATION ............................................................................................... xii

ABBREVIATIONS ..................................................................................................................... xiii

1.0 MYTHS AND FACTS ABOUT EARTHING ...................................................................... 1

1.1 Myth-1: “Earth, Earthing and Earth Electrode, all these 3 words convey the same meaning. ............................................................................................... 1

1.2 Myth-2: “Only Connecting exposed conductive part of an electrical equipment to the earth electrode in soil will protect against electric shock”. ..................................................................................................................... 1

1.3 Myth-3: “When earth fault occurs the fault current goes into the earth and natural earth serves as a return path for fault current”. ................. 2

1.4 Myth-4: “Entire fault current will always flow through least resistance path”. ........................................................................................................................ 3

1.5 Myth-5: ‘Grounding’ implies connection of current carrying parts to

Earth, like transformer or generator neutral and ‘Earthing’ implies connection of non-current carrying parts to Earth, like metallic enclosures”. ............................................................................................................ 3

1.6 Myth-6: “The more earth electrodes in soil (Separate equipment earth electrode), the better system”. .......................................................................... 3

1.7 Myth-7:“Copper Earth Electrodes are better than GI or Steel Earth Electrodes”. ............................................................................................................. 4

1.8 Myth-8:“Plate Earthing is better than Pipe Earthing as it offers less resistance”. ............................................................................................................. 5

1.9 Myth-9: “Deeper the earth pit and longer the earth pipe/rod, lesser will be the resistance”. ................................................................................................ 6

1.10 Myth-10: Adding more water will result in less resistance of earth pit.7

1.11 Myth-11: Adding more salt will result in less resistance of Earth. ........ 7

1.12 Myth 12: Only high voltage lines pose a fatal danger and low voltage i.e. 240V AC domestic supply is not fatal. .................................................... 8

DRAFT

v

1.13 Myth 13: All Rubber shoes/Sleepers/gloves can protect from electric shock. ....................................................................................................................... 9

1.14 Myth 14: Wood is an insulator and wooden ladders cannot conduct electricity. ................................................................................................................ 9

1.15 Myth 15: Neutral& body of every transformer/DG and the metallic parts not intended as conductors each requires two separate and distinct connections to earth electrodes in soil. ........................................... 9

2.0 PURPOSE OF EARTHING ................................................................................................ 12

3.0 EARTHED AND UNEARTHED SYSTEMS .................................................................... 12

4.0 EQUIPMENT EARTHING ................................................................................................. 13

4.1 Earthing of Equipment using Current ...................................................................... 13

4.2 Voltage Exposure .................................................................................................... 14

4.2.1 Avoidance of Thermal Distress ................................................................. 14

4.2.2 Preservation of System Performance ......................................................... 14

4.2.3 Earthing &Protective Conductor ............................................................... 14

5.0 SYSTEMEARTHING ......................................................................................................... 15

6.0 TYPES OF SYSTEM EARTHING..................................................................................... 16

6.1 TN System ............................................................................................................... 17

6.1.1 TN-S System ............................................................................................. 17

6.1.2 TN-C System ............................................................................................. 18

6.1.3 TN-C-S System ......................................................................................... 19

6.2 TT System ................................................................................................................ 20

6.3 IT System ................................................................................................................. 21

7.0 EQUIPOTENTIAL BONDING .......................................................................................... 22

7.1 Protective Equipotential Bonding ............................................................................ 23

7.2 Supplementary Equipotential Bonding .................................................................... 23

7.3 Main Earthing Terminal (MET) .............................................................................. 24

7.4 Typical Schematic of Earthing and Protective Conductors ..................................... 25

7.5 Extraneous Conductive Parts ................................................................................... 26

(Ref IS 3043:2018 para 23.2.3 ) .......................................................................................... 26

7.6 Exposed Conductive Parts ....................................................................................... 26

(Ref IS 3043:2018 para 23.2.4) ........................................................................................... 26

7.6.1 Exposed conductive parts not required to be earthed ................................ 26

8.0 EARTH FAULT PROTECTION IN LV INSTALLATIONS ............................................ 27

8.1 Protection against Indirect Contact .......................................................................... 28

8.2 Earthed Equipotential Bonding and Automatic Disconnection of the Supply ........ 29

8.2.1 Disconnecting Times for Different Touch Voltages ................................. 30

8.3 Requirement for Automatic disconnection in TN System ....................................... 30

DRAFT

vi

8.4 Requirement for Automatic disconnection in TT System ....................................... 31

8.5 Requirement for Automatic disconnection in IT System ........................................ 31

9.0 EARTH FAULT LOOP IMPEDANCE .............................................................................. 31

9.1 Fault Loop in TN-S System ..................................................................................... 32

9.2 Fault Loop in TN-C System .................................................................................... 32

9.3 Fault Loop in TN-C-S System ................................................................................. 33

9.4 For TT System ......................................................................................................... 33

10.0 EARTHING OF INSTALLATIONS .................................................................................. 34

10.1 Earthing in Standby Generating Plants (Ref - IS 3043:2018 para 29) .................... 34

10.1.1 Unearthed Generators (Rating Below 10 kW) Supplying a Fixed Installation

................................................................................................................... 34

10.2 Unearthed Generators Supplying a Mobile or Transportable Unit .......................... 35

10.3 Earthing in Building ................................................................................................ 36

11.0 EARTH ELECTRODES ..................................................................................................... 38

11.1 Effect of Shape on Electrode Resistance ................................................................. 38

11.2 Multiple Earth Electrodes ........................................................................................ 38

11.3 Common Types of Earth Electrodes ........................................................................ 39

11.3.1 Plate electrode ........................................................................................... 39

11.3.2 Pipes or Rods electrode ............................................................................. 39

11.3.3 Artificial treatment of soil ......................................................................... 42

11.3.4 Strip or Conductor Electrodes ................................................................... 42

11.4 Other Means of Earthing ......................................................................................... 43

11.4.1 Cable Sheaths ............................................................................................ 43

11.4.2 Structural Steelwork as Earth Electrodes .................................................. 43

11.4.3 Reinforcement of Piles .............................................................................. 44

11.4.4 Water Pipes ................................................................................................ 44

11.5 Connections to Earth Electrodes .............................................................................. 44

12.0 EARTHING SYMBOLS ..................................................................................................... 45

13.0 VOLTAGE GRADIENT AROUND EARTH ELECTRODES .......................................... 46

14.0 CURRENT DENSITY AT THE SURFACE OF EARTH ELECTRODE ......................... 46

15.0 BASICS OF ELECTRIC CIRCUIT .................................................................................... 47

15.1 Simple Open and Closed Circuit ............................................................................. 47

15.2 Open and Closed Circuit with Earth ........................................................................ 47

16.0 ELECTRIC SHOCK ........................................................................................................... 48

16.1 Range of Tolerable Current ..................................................................................... 48

16.2 Effect of Frequency ................................................................................................. 49

16.3 Effect of Magnitude and Duration of Current Passing through Body ..................... 49

DRAFT

vii

16.4 Resistance of Human Body ..................................................................................... 49

ANNEXURE-I .............................................................................................................................. 51

EARTHING TERMINOLOGY .................................................................................................... 51

(Ref IS 3043:2018 & IS 732:2019) ............................................................................................... 51

i. Arc-Suppression Coil (Peterson Coil) .............................................................. 51

ii. Bonding Conductor .......................................................................................... 51

iii. Class 0 Equipment ............................................................................................ 51

iv. Class I Equipment ............................................................................................. 51

v. Class II Equipment ........................................................................................... 51

vi. Class III Equipment .......................................................................................... 51

vii. Principal Characteristics of Equipment and Precautions for Safety ................. 51

viii. Dead 52

ix. Double Insulation ............................................................................................. 52

x. Reference Earth ................................................................................................ 52

xi. Earthing System ................................................................................................ 52

xii. Global Earthing System .................................................................................... 52

xiii. Earth Electrode ................................................................................................. 52

xiv. Earth Grid ......................................................................................................... 52

xv. Earth Electrode Resistance ............................................................................... 52

xvi. Earth Fault Loop Impedance ............................................................................ 52

xvii. Earth Leakage Current ...................................................................................... 52

xviii. Earthed Concentric Wiring ............................................................................... 53

xix. Earthing Conductor .......................................................................................... 53

xx. Electrically Independent Earth Electrodes ....................................................... 53

xxi. Equipotential Bonding ...................................................................................... 53

xxii. Exposed Conductive Part ................................................................................. 53

xxiii. Extraneous Conductive Part ............................................................................. 53

xxiv. Final Circuit ...................................................................................................... 53

xxv. Functional Earthing .......................................................................................... 53

xxvi. Live Part ........................................................................................................... 53

xxvii. Main Earthing Terminal ................................................................................... 53

xxviii. Neutral Conductor ............................................................................................ 54

xxix. PEN Conductor ................................................................................................. 54

xxx. Portable Equipment .......................................................................................... 54

xxxi. Potential Gradient (At a Point) ......................................................................... 54

xxxii. Earth Potential .................................................................................................. 54

xxxiii. Earth Potential Rise .......................................................................................... 54

DRAFT

viii

xxxiv. Hot Site ............................................................................................................. 54

xxxv. Transferred Potential ........................................................................................ 54

xxxvi. Protective Conductor ........................................................................................ 54

xxxvii. Reinforced Insulation ....................................................................................... 55

xxxviii. Residual Current Device ................................................................................... 55

xxxix. Residual Operating Current .............................................................................. 55

xl. Resistance Area (for an Earth Electrode only) ................................................. 55

xli. Simultaneously Accessible Parts ...................................................................... 55

xlii. Supplementary Insulation ................................................................................. 55

xliii. Switchgear ........................................................................................................ 55

xliv. Voltage, Nominal ............................................................................................. 55

xlv. Touch Voltage .................................................................................................. 55

xlvi. Prospective Touch Voltage ............................................................................... 56

xlvii. Step Voltage ..................................................................................................... 57

xlviii. Equipotential Line or Contour .......................................................................... 57

xlix. Mutual Resistance of Earthing Electrodes ....................................................... 57

l. Earth Grid ......................................................................................................... 57

li. Earth Mat .......................................................................................................... 57

REFERENCES ............................................................................................................................. 58

DRAFT

ix

LIST OF FIGURES


Figure 1: Effect of Length of Pipe Electrode on Calculated Resistance for Soil Resistivity of

100m (Assumed uniform) ............................................................................................ 6

Figure 2: Variation of Soil Resistivity with Moisture Content ...................................................... 7

Figure 3: Variation of Soil Resistivity with Salt (NaCl) Content, Clay Soil Having 3 Percent

Moisture .......................................................................................................................... 8

Figure 4: Wrong practice of connecting every appliance to two separate earth electrodes. .......... 9

Figure 5: Wrong practices of separate earth electrodes in soil .................................................... 11

Figure 6: Right practice of Earthing ............................................................................................. 11

Figure 7: Marking of Earth Conductors ....................................................................................... 15

Figure 8: Indicative supply system based on system earthing ..................................................... 16

Figure 9: TN-S System ................................................................................................................. 17

Figure 10: TN-S System for 230V Single Phase ........................................................................... 17

Figure 11: TN-S System for 415 V three-phase domestic commercial supply .............................. 18

Figure 12: TN-C System ................................................................................................................ 18

Figure 13: Neutral breakage in TN-C System ................................................................................ 19

Figure 14: TN-C-S System ............................................................................................................. 19

Figure 15: Multiple Earthing of PEN Conductor in TN-C-S System ............................................ 20

Figure 16: TT System ..................................................................................................................... 21

Figure 17: IT System ...................................................................................................................... 21

Figure 18: Equipotential bonding with all exposed/extraneous conductive parts .......................... 22

Figure 19: Equipotential Bonding with all exposed/extraneous conductive parts ......................... 23

Figure 20: Equipotential Bonding with Supplementary Bonding .................................................. 24

Figure 21: Main Earthing Terminal (MET) ................................................................................... 24

Figure 22: Earthing Arrangements and Protective Conductors ..................................................... 25

Figure 23: Indirect Contact in case of earth fault ........................................................................... 27

Figure 24: A Non Conductive Location ......................................................................................... 28

Figure 25: Fault loop impedance circuit in TN-S system .............................................................. 32

Figure 26: Fault loop impedance circuit in TN-C system .............................................................. 32

Figure 27: Fault loop impedance circuit in TN-C-S system .......................................................... 33

Figure 28: Fault loop impedance circuit in TT system .................................................................. 33

Figure 29: Single Low Voltage Standby Generator (Without paralleling Facility) ....................... 34

Figure 30: Small low voltage generator supplying a fixed installation ......................................... 35

Figure 31: Small unearthed generator supplying a mobile or transportable unit ........................... 36

Figure 32: Example of Earthing Arrangements and Protective Conductors .................................. 37

DRAFT

x

Figure 33: Sphere of Influence Earth Electrode ............................................................................. 38

Figure 34: Spheres of Multiple Earth Electrodes ........................................................................... 38

Figure 35: Copper Plate Electrode ................................................................................................. 39

Figure 36: Pipe Electrode ............................................................................................................... 40

Figure 37: Copper Bonded Steel rod Electrode ............................................................................. 40

Figure 38: Typical arrangement of pipe electrode (Ref fig 25 of IS 3043:2018) .......................... 41

Figure 39: Effect of Length of Strip or Conductor Electrodes In Calculated Resistance For Soil

Resistivity Of 100 ΩM (Assumed Uniform) ................................................................ 43

Figure 40: Earthing of Foundation Reinforcement ........................................................................ 44

Figure 41: Incomplete circuit or open circuit, no current will flow in above condition. ............... 47

Figure 42: Complete circuit or close circuit, current will flow in this condition according to the

source voltage and load resistance. ............................................................................... 47

Figure 43: Incomplete circuit or open circuit, no current will flow in above condition. ............... 48

Figure 44: Complete circuit or close circuit, current will flow in this condition according to the

source voltage and circuit resistance. ........................................................................... 48

Figure 45: Illustration of body resistance for different part. .......................................................... 50

Figure 46: Step and Touch Voltages .............................................................................................. 56

LIST OF TABLES


Table 1: Comparison between Earthed and Unearthed System ................................................. 12

Table 2: Disconnecting Times for Different Touch Voltages .................................................... 30

Table 3 : Effect of Magnitude and Duration of Current Passing through Body .......................... 49

Table 4: Equipment Characteristics and Safety Precautions ...................................................... 51

DRAFT

xi

ISSUE OF CORRECTION SLIP

The correction slips to be issued in future for this handbook will be numbered as follows:

CAMTECH/ EL/2020-21/Earthing Vol.1/1.0/C.S. # XX date---

Where “XX” is the serial number of the concerned correction slip (starting from 01

onwards).

CORRECTION SLIPS ISSUED

Sr. No. Date of Issue Page no. & Item no. modified Remarks

DRAFT

xii

DISCLAIMER

It is clarified that the information given in this booklet does not supersede

any existing provisions of Indian Standards (IS) on the subject, related

matters, and other existing provisions laid down by the Railway Board,

RDSO. This is not a statuary document and instructions given are for the

purpose of guidance only. If at any point contradiction is observed, then

Indian Standards, regulations issued by Government bodies, Railway

Board/RDSO guidelines shall be referred.

.

OBJECTIVE OF PUBLICATION

To prepare guidelines which can educate the IR engineers and

Technicians dealing with Low voltage electrical installations and other IR

officials about various provisions given in IS standards pertaining to IS

3043-2018, IS 732:2019.

DRAFT

xiii

ABBREVIATIONS

Abbreviation Full Form

AC Alternating Current

CEA Central Electricity Authority

CEAR Central Electricity Authority Regulations

CNE Combined Neutral and Earth

DB Distribution Board

DC Direct Current

EE Earth electrode

ELCB Earth leakage Circuit Breaker

IE Indian Electricity

IEC International Electro-technical Commission

IS Indian Standard

LPS Lightning Protection System

MDB Main Distribution Board

MET Main Earthing Terminal

N Neutral

OCPD Over Current Protective Device

PEN Neutral and Protective conductor combined

PME Protective Multiple Earthed

PNB Protective Neutral Bonding

RCD Residual Current Device

SEBT Supplementary Equipotential Bonding Terminal

SPD Surge Protection Device

T Terre (French word for earth)

CAMTECH/ EL/ 2020-21/ Earthing-Vol.-1/ 1.0 1

Earthing system & Protection in Low Voltage Installation March, 2021

1.0 MYTHS AND FACTS ABOUT EARTHING

It is generally observed that there are several myths and perceptions about earthing in the

minds of IR personnel dealing with low voltage electrical installations, which are either

not true or partially true. Therefore, some of the commonly noticed myths have been

enumerated below these myths have then been discussed with the facts in a summarised

way. The other pertaining points have been covered in various other chapters.

1.1 Myth-1: “Earth, Earthing and Earth Electrode, all these 3 words convey the same meaning.

Fact: Not True

The earth, earthing and earth electrodes convey different meaning, earth is general mass

of mother earth while earth electrode is an independent system provided to give electrical

connection to mother earth (soil). The Earthing is an elaborate network to connect all

exposed conductive parts of any equipment/installation to earth potential through MET

(Main Earthing Terminal). Incidentally, definitions given in Indian Standard (IS) are

summarized below:

“Earth means the conductive mass of the earth, whose electric potential at any point is

conventionally taken as zero”.(Ref.: Para no. 3.60 of IS 732:2019)

“Earthing means connection of the exposed conductive parts of an installation to the

Main Earthing Terminal (MET) of that installation”.(Ref.: Para no. 3.66 of IS 732:2019)

“Earth Electrode means a conductor or group of conductors in intimate contact with

and providing an electrical connection to earth. (Ref.: Para no. 3.7 of IS 3043:2018)

They can include rods, plate, strip, steel reinforcing bars, sheaths of cables etc.”.

1.2 Myth-2: “Only Connecting exposed conductive part of an electrical equipment to the earth electrode in soil will protect against electric shock”.

Myth Explanation: - In general household/commercial wiring the earth wire is also

running along with phase and neutral wires. The conductive exposed part/body of

electrical equipment which is connected to earth through the earth wire and further earth

wire is connected to the earth electrode. It is pre-assumed that if there is any earth fault

happens through contact of phase/live conductor to the conductive exposed part/body of

the equipment/installation, the earth fault current will flow through the earth wire to earth

electrode then to earth mass/back to the source (Transformer/Generator) and this way

protective device (Fuse, MCB, etc.) will operate and protect person from the electric

shock.

MET(Main Earthing Terminal):- The terminal or bar (which is the equipotential

bonding conductor) provided for the connection of protective conductors and the

conductors of functional earthing, if any, to the means of earthing. (Ref.: Para no. 3.21

of IS 3043:2018).

Exposed conductive part:- A conductive part of equipment which can be touched

and which is not alive part but which may become live under fault conditions. Like

panel Body. (Ref.: Para no. 3.16 of IS 3043:2018).



Fact: Not true

Protective equipotential bonding and automatic disconnection of supply with in

permissible time can only prevent electric shock.

It is a very common myth that system is healthy if connected to earth electrode with least

earth pit resistance but this may not be true in practical condition every time to have a

low resistance earth pits such that fault current magnitude can cause tripping of OCPD

(Over Current Protective Device) within permissible time. Because of this reason only

connection of exposed conductive parts of electrical equipment to earth electrode in soil

will not protect from an electric shock.

Protective equipotential bonding1 that is connecting of the exposed conductive parts and

extraneous conductive parts with Main Earthing Terminal. Whenever earth faults occur a

voltage will appear between the exposed conductive parts and extraneous conductive

parts in the location served by the installation concerned, but the application of protective

equipotential bonding minimizes these touch voltage reaches a harmful value.

In case of earth fault if the fault current is less than the rated breaking current of

protective device (OCPD like MCB), then it will not trip. The automatic disconnection of

supply system should be designed considering fault loop impedance2, which is the

impedance/resistance encountered by current under fault condition i.e.

impedance/resistance from fault point to neutral point of the source

(Transformer/generator)shall be such that it will disconnect the supply to faulty

equipment by detection of fault current and leakage current through use of suitably rated

OCPD (Over Current Protective Device like fuse, MCB, etc.) and RCD (Residual

Current Device like RCCB) within stipulated time.

1.3 Myth-3: “When earth fault occurs the fault current goes into the earth and natural earth serves as a return path for fault current”.

Fact: Partially True

As a Fact, Current will always return back to its source (Transformer/generator).In

any electrical system, during earth fault, the fault current will not get absorbed into the

earth and it will go back to the source only either via return conductor or through mother

earth.

In case of earth fault, the fault current will flow back to the source through the return

conductor (protective earth & neutral conductor) in case of TN-C-S system. As IS

3043:2018 stipulates several types of earthing systems like TN(TN-S, TN-C, TN-C-S)

TT, & IT in which TN-C-S is most common earthing system used in India for LV

installations as mandated under statutory provisions in para-4 of IS 3043:2018, other than

some specialized requirements. Types of earthing systems3are further explained in

detail in this booklet.

As per IS 3043-2018 para 4 of foreword, “the earth now rarely serves as a part of the

return circuit but is being used mainly for fixing the voltage of system neutrals”. It is

important to note that natural earth is a very poor conductor of electric current. The

1Refer to para 7.0 of this booklet





typical resistivity of the general mass of earth is about 100 Ohm-m. Compare this with

the resistivity of Copper, which is, 1.7 x 10-8

Ohm-m (0.017 Ohm-mm2/m) and that of

GI, which is, 1 x 10-7

Ohm-m (0.1 Ohm-mm2/m).This shows that, natural earth is

much more resistive than Copper or GI.

The purpose of earth connection is to improve service continuity and to avoid damage to

equipment and danger to human life.

1.4 Myth-4: “Entire fault current will always flow through least

resistance path”.

Fact: Partially true

Current does not only take the path of least resistance. Current will flow through all

possible closed paths back to its source, only the magnitude of current will depend on the

impedance of that particular closed path. For a lower impedance path, current will be

higher compared to a higher impedance path.

1.5 Myth-5: ‘Grounding’ implies connection of current carrying parts to Earth, like transformer or generator neutral and ‘Earthing’ implies connection of non-current carrying parts to Earth, like metallic

enclosures”.

Fact: Not true.

Earthing and Grounding are synonym to each other.

The terms ‘earthing’ and ‘grounding’ are used synonymously. This is also mentioned

in para8 of ‘Foreword’ of IS 3043-2018.

The different nomenclature is due to the usual conflicting usage of English language

between the Americans & the British.

British termed it as ‘earthing’, while the Americans termed it as ‘grounding’. IEC &

IS Standards referring as ‘earthing’, while IEEE & ANSI Standards referring as

‘grounding’.

1.6 Myth-6: “The more earth electrodes in soil (Separate equipment earth electrode), the better system”.

Fact: Not true.

More earth electrodes in soil i.e. separate earth electrode for each equipment, can

cause more problems than it solves. Adding more earth electrodes in soil can actually

expose the equipment/building to current from nearby lightning strikes or fault in

Electrical power system of utilities and this may cause equipment failure by providing a

path for that high magnitude current spike to travel through electrical and electronics

equipment. Separate earth electrode for individual equipment is a misconception which is

explained in detail in 1.15.

When lightning strike happens, there is rise in earth potential near an electrode in that

area, and that may provide an additional path between earth electrodes and earth

conductors for lightening/fault current to travel through the equipment which may

damage equipment. This may be avoided by connecting earthing of all equipment



through protective earth conductor to Main Earthing Terminal. which is further

connected to the earth electrode/return conductor depending on type of earthing system.

1.7 Myth-7:“Copper Earth Electrodes are better than GI or Steel Earth

Electrodes”.

Fact: Not true.

The type of Metal used has no significance on the earth electrode resistance.

If we, consider a plate electrode, according to Cl. 14.2.1 of IS3043-2018, the

approximate resistance to earth of a plate can be calculated from:

where

= resistivity of the soil in -m (assumed uniform), and

A = area of both sides of the plate (in m2).

Similarly, for Rod or Pipe Electrodes, according to Cl. 14.2.2 of IS3043-2018, the

resistance of a pipe or rod electrode is given by:

Where,

ρ = resistivity of the soil, in Ω-m (assumed uniform).

l = length of rod or pipe, in cm,

d = diameter of rod or pipe, in cm, and

And, for Strip or Conductor Electrodes, according to Cl. 14.2.3 of IS3043-2018,

the resistance of a strip or conductor electrode is given by:

where,

ρ = resistivity of the soil, in Ω-m (assumed uniform),

l = length of the Strip in cm, and

d = width (strip) or twice the diameter (conductors) in cm.

As can be seen from the above formulae, only the resistivity of the soil and the

physical dimensions of the electrode play a major role in determining the electrode

resistance to earth. The material resistivity is not considered anywhere in the above

formulae. Hence, irrespective of the material of construction of the earth electrode,

any material of given dimensions would offer the same resistance to earth.

However, in highly corrosive areas, it is recommended to use Cu or Cu coated

electrodes which have better resistance to corrosion as compared to GI/Steel

electrode.



1.8 Myth-8:“Plate Earthing is better than Pipe Earthing as it offers less

resistance”.

Fact: Not true.

A pipe, rod or strip has a much lower resistance than a plate of equal surface area.

It is common believe that plate earthing is better over Pipe or Rod or Strip Earthing

because more surface area means less resistance.

where,

ρ = resistivity of the conductor material in Ω-m,

l = length in m, and

A = cross sectional area m2.

But this is not correct formula to calculate earth electrode resistance.

To examine this, let us consider a plate electrode of size 60 cm x 60 cm x 3.15mm

thick. Assuming a soil resistivity of 100 Ohm-m, the resistance of this electrode to earth

as per following formula,

Now, consider a Pipe Electrode of 50 mm Diameter and 400 cm Long. Assuming a soil

resistivity of 100 Ohm-m, the resistance of this electrode as per following formula to

earth:

As can be seen from the above calculation the Pipe electrode (surface area 0.63 m2)

offers a much lesser resistance than even a plate electrode of more surface area (0.72

m2).

IS 3043-2018 also acknowledges this fact vide Cl. 14.1, wherein it states that ‘a pipe, rod

or strip has a much lower resistance than a plate of equal surface area. The resistance is

not, however, inversely proportional to the surface area of the electrode.

However for higher current density requirements plate earthing may be preferred over

pipe/rod earthing.



1.9 Myth-9: “Deeper the earth pit and longer the earth pipe/rod, lesser

will be the resistance”.


As stated in Cl. No. 14.2.2 of IS 3043-2018, “that the resistance to earth of a pipe or rod

electrode diminishes rapidly with the first few feet of driving, but less so at depths

greater than 2 to 3 m in soil of uniform resistivity”.

Figure 1: Effect of Length of Pipe Electrode on Calculated Resistance for Soil

Resistivity of 100m (Assumed uniform)

As can be seen from the graph given above (Cl. No. 14.2.2 of IS 3043-2018), after about

4m depth, there is no appreciable change in resistance to earth of the electrode.

Number of rods/ pipes in parallel are to be preferred to a single long rod/pipe. When a

number of rods/ pipes are connected in parallel, the resistance is practically proportional

to the reciprocal of the number of electrode connected, provided each electrode is

situated outside the resistance area of any other electrode.

Deeply driven rods are, however, effective where the soil resistivity decreases with depth

or where substrata of low resistivity occur at depths greater than those with rods, for

economic reasons, are normally driven.

The earth electrode resistance value from a 4 meters long earth

electrode cannot be replaced by using two electrodes connected in

parallel each of 2 meters length (considering uniform soil resistivity)



1.10 Myth-10: Adding more water will result in less resistance of earth

pit.


The resistance to earth of a given earth electrode depends upon the electrical resistivity of

the soil in which it is installed. Moisture content is one of the controlling factors of earth

resistivity.

Figure 2: Variation of Soil Resistivity with Moisture Content

As can be seen from the above graph (Cl. No. 13.6, fig 19 of IS 3043-2018), that above

20 percent moisture content, the resistivity is very little affected. Below 20 percent, the

resistivity increases abruptly with the decrease in moisture content. If the moisture

content is already above 20 percent, there is no point in adding barrels of water into the

earth pit as this may not help in change in resistance of earth electrode.

1.11 Myth-11: Adding more salt will result in less resistance of Earth.


As a traditional practice whenever an earth pit is commissioned a homogenous layer of

coke/charcoal, salt is added surrounding the electrode as per figure 14 of IS 3043:1987.

But now As per latest guidelines of IS 3043:2018figure no. 25 (Figure 38 of this

booklet), only homogenous layer of common earth is to be used. However earth

enhancing material may be used where soil resistivity is high.

As a common practice, during maintenance of earth pits in order to reduce the earth

electrode resistance more salt is added in the pit in a belief that it will reduce the earth

electrode resistance drastically.

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Moisture in Soil, Percent

400

300

200

100

0

Res

isti

vit

y o

f S

oil

in O

HM

Met

re



The reduction in soil resistivity effected by the salt content is shown in the curve given

below (Cl. No. 13.8.3 fig-20 of IS 3043-2018)

Figure 3: Variation of Soil Resistivity with Salt (NaCl) Content, Clay Soil Having 3

Percent Moisture

The salt content is expressed in percent by weight of the moisture content in the soil. As

can be seen from the above, the curve flattens off at about 5 percent. A further increase

in salt content will give a very little decrease in soil resistivity. So, there is no point

again, in just filling up the pit with salt.

For example, if we consider volume of soil is 1M3(1440kg.), the moisture content at 10

percent will be about 144 kg. (10 percent of 1440 kg). The salt content shall be 5% of

this (i.e.) 5% of 144kg, that is, about 7.2kg. So, water addition should be about 144 kg

and salt addition shall be about 7.2 kg per cubic metre of dry soil. Any further additions

will not give appreciable benefits.

Further discussion on artificial treatment of soil is given in para

1.12 Myth 12: Only high voltage lines pose a fatal danger and low

voltage i.e. 240V AC domestic supply is not fatal.

Fact: “Not true”.

400

300

200

100

Res

isti

vit

y o

f S

oil

in

OH

M M

etre

350

250

150

50

0 2 4 6 8 10 12 14 16

Percent of Salt in Moisture

Bentonite or similar earth enhancing material may be advantageous in

rocky terrain. Bentonite packing will increase the contact efficiency with

the general mass of ground.

The possible corrosive effect of the salt should be considered which may

also reduce the life of the earth electrodes and connections.



Voltage creates the current flow through the body depending upon the body resistance.

Any voltage above 50 Volt AC can be fatal to human being. As per para 31.1.1.1 of IS

3043:2018 conventional touch voltage limit is 50V AC. Details of various effects of

current on human body are explained further in this book.

1.13 Myth 13: All Rubber shoes/Sleepers/gloves can protect from electric

shock.


Not all rubber shoes/sleepers/gloves provide full protection from electric shock. Proper

voltage-rated rubber gloves and shoes that are certified as electrically safe for that rated

voltage can only protect against electric shock.

1.14 Myth 14: Wood is an insulator and wooden ladders cannot conduct electricity.


Wood is a poor conductor but it can conduct electricity, especially when wet or when it

has metal brackets. Using a wooden ladder when working with live electrical wiring is

not a guarantee of safety. Additional guidelines, such as equipotential bonding of all

metal parts including live wires and body of equipment, wearing voltage insulated

gloves, shoes and working with de-energized circuits can only give protection against

electric shock.

1.15 Myth 15: Neutral& body of every transformer/DG and the metallic parts not intended as conductors each requires two separate and

distinct connections to earth electrodes in soil.

Myth Explanation

There is a practice of connecting transformer (or DG) neutral terminal by two separate

earth electrodes in soil. Body of transformer (or DG) is also connected by two separate

earth electrodes in soil as per the interpretation of rule no 41 of CEAR (MEASURES

RELATING TO SAFETY AND ELECTRIC SUPPLY)-2010 as figure given below.

Figure 4: Wrong practice of connecting every appliance to two separate earth electrodes.

Fact: Not True

Due to this misinterpretation every electrical apparatus is connected to earth electrode in

soil by 2 separate earth pits. This misinterpretation is probably the biggest threat for



electrical safety in India. Neither CEAR-2010 nor IS standards recommend these

practices.

Before going further make sure you have read myth-1.The distinction between earth,

earthing and earth electrode has already been elaborated at Myth-1.

Selective part of CEAR 2010, Regulation 41 are reproduced below to understand this

myth:

“The following conditions shall apply to the connection with earth of systems at voltage

normally exceeding 125 V but not exceeding 650 V, namely:”

(i) Neutral conductor of a 3-phase, 4-wire system and the middle conductor of a 2-

phase, 3-wire system shall be earthed by not less than two separate and distinct

connections with a minimum of two different earth electrodes or such large number

as may be necessary to bring the earth resistance to a satisfactory value both at the

generating station and at the sub-station.

Interpretation:- It is the Neutral conductor (not neutral point) which is required to

be connected by not less than two separate and distinct connections with a

minimum of two different earth electrodes.

(iii) In the case of a system comprising electric supply lines having concentric cables,

the external conductor of such cables, shall be earthed by two separate and distinct

connections with earth.

Interpretation:- It says the armour and/or sheath of such cables shall be earthed

by two separate and distinct connections with earth, not with the two separate and

distinct earth electrodes.

(iv) The frame of every generator, stationary motor, portable motor, and the metallic

parts, not intended as conductors, of all transformers and any other apparatus used

for regulating or controlling electricity, and all electricity consuming apparatus, of

voltage exceeding 250 V but not exceeding 650 V shall be earthed by the owner by

two separate and distinct connections with earth.

Interpretation:- It says the metallic frame of every generator and the metallic

parts, not intended as conductors, shall be earthed by two separate and distinct

connections with earth, not with the two separate and distinct earth electrodes.

(v) Neutral point of every generator and transformer shall be earthed by connecting it

to the earthing system by not less than two separate and distinct connections.

Interpretation:- It is Neutral point not neutral conductor which is required to be

connected by not less than two separate and distinct connections with earthing

system, not with the two separate and distinct earth electrodes.

As per above, the Transformer (or DG) body need to be connected to earth and not to

specifically separate earth electrodes in soil. The regulation never recommended to

connect Transformer (or DG) Neutral and body to separate earth pits. This wrong

practice followed is merely a misinterpretation of the regulation.

However, wrong practices create accidents during fault. Following figures explain the

above discussion for better understanding.



WRONG CONCEPT

Typical Indian practice. This wrong

practice may lead to accidents,

deterioration of insulation, reduced

life, malfunctioning of electronics.

WRONG CONCEPT

Independent earth electrodes

resulting accidents, deterioration of

insulation, reduced life,

malfunctioning of electronics.

WRONG CONCEPT

Separate earth electrodes connected

as a grid under soil, resulting

accidents, deterioration of

insulation, reduced life,

malfunctioning of electronics,

additional problems due to

circulating currents.

Figure 5: Wrong practices of separate earth electrodes in soil

Figure 6: Right practice of Earthing

GOOD PRACTICE

This is correct practice to connect all earthing conductors and earth electrodes to

create an equipotential bonding among all for safe and maximum life of electrical

and electronic system.



2.0 PURPOSE OF EARTHING (Ref: IS 3043:2018, 11.1.3)

Earthing of an electrical system or installation is provided for the reasons of safety.

The earth connection improves service continuity, avoids damage to equipment and

danger to human life.

Earthing also reduces the risk of a person in the vicinity of earthed facilities being

exposed to the danger of critical electric shock.

Earthing in a substation is done to provide as nearly as possible a surface at a uniform

potential and as nearly as zero or absolute earth potential. The purpose of this is to

ensure that all parts (metal parts/exposed conductive parts) of apparatus/equipment

other than live parts shall be at uniform/earth potential. This will also ensure that

user/operator will always remain at earth potential at all times.

Earthing also provides stable platform for operation of sensitive electronic equipment

i.e., to maintain the voltage at any part of an electrical system at a known value so as

to prevent over current or excessive voltage on the appliances or equipment.

Earthing also provides a low earth fault loop impedance to facilitate automatic

disconnection of supply in the event of a fault to exposed conductive parts of

equipment/ apparatus.

Earthing will also help to limit the rise in potential (touch voltage) of non-current

carrying metal parts with respect to earth under earth-fault conditions to a value

which is not harmful for the safety of persons/animals in contact/proximity to such

metal parts.

Earthing also provides safe path to dissipate lightning current to earth.

3.0 EARTHED AND UNEARTHED SYSTEMS (Cl. No.5 of IS 3043-2018)

Table 1: Comparison between Earthed and Unearthed System

Earthed systems Unearthed system

Earthed systems are designed and installed

so that the protective devices provided in

the circuit will operate and remove the

faulty circuit from the system in case of any

earth fault.

In this way the faulty circuit can be isolated

from rest of the system even in case of first

earth fault before any damage.

In case of un-earthed system, on occurring first

earth fault, neither circuit performance will be

affected nor there will be any indication and in

general there will also be no service interruption.

The occurrence of a second earth fault on a

different phase before the first fault is cleared,

does result in an outage.

It is experienced that multiple earth faults

are rarely, if ever, experienced on earthed

neutral systems.

The longer a earth fault is allowed to remain on

an unearthed system, greater is the probability of

a second one occurring in another phase, causing

failure and repairs are required to restore

service.

In earthed system, any earth fault is

detected and isolated by automatic

disconnection of supply.

For unearthed system, an organized maintenance

programme is extremely necessary so that earth

fault if any is traced and removed soon after



Earthed systems Unearthed system

detection.

In earthed system, the protection of

personnel and property from hazards are

ensured thorough earthing of equipment

and structures.

Proper earthing results in less chance of

accidents/electrocution to personnel.

In unearthed system also, the protection of

personnel and property from hazards are

required thorough earthing of equipment and

structures.

An earth fault does not open the circuit in an

unearthed system, some means of detecting the

presence of an earth fault requires to be installed

like Insulation monitoring, leakage current

monitoring etc.

4.0 EQUIPMENT EARTHING (Ref: IS 3043:2018, Para 12.0)

Earthing of non-current carrying metal work & conductor which is essential to the safety

of human life, animals and property is known as Equipment Earthing or Protective

Earthing. (Ref IS 3043:2018 Forward para 6)

“Equipment earthing or protective earthing of low voltage installation as per IS732:2019

is provided to limit the potential with respect to the general mass of earth of non-current

carrying metal works associated with equipment, apparatus and appliance connected to

the system”.( Ref IS 3043:2018 1.1 (a))

The basic objectives of equipment earthing are:

a) “To ensure freedom from dangerous electric shock/voltages exposure to persons in

the area”.

b) “To provide adequate earth fault current carrying capability, both in magnitude and

duration, permitted by the protective system without creating a fire or explosive

hazard to building/equipment”.

4.1 Earthing of Equipment using Current

(Ref: IS 3043-2018, para 11.1.4)

Class I appliances are usually made of

metal, have three cables, it has a metal

Earth pin (Earth wire of the equipment

which comes to the earth pin of 3 pin

plug top). Appliances under Class I

have two levels of protection, the basic

insulation and the earth connection.

Accessible/exposed metal parts of

Class-I equipment/appliances that may

become live in the event of an

insulation failure, shall be permanently

and reliably connected to an earthing

terminal within the appliance or to the earthing contact of the appliance inlet. Ex. Iron,

refrigerators, microwaves etc.

Do not connect Earthing terminal and earthing contact of the equipment

to the neutral terminal.

Class I Equipment (Ref IS 9049:1980)

Equipment in which protection against

electric shock does not rely on basic

insulation only, but which, includes an

additional safety, precaution in such a way

that means are provided for the connection of

accessible conductive parts to the protective

(earthing) conductor in the fixed wiring of the

installation in such a way that accessible

conductive parts may not become live in, the

event of a failure of the basic insulation.



A Class II (double insulated) electrical

appliance is one which has been

designed in such a way that it does not

require a safety connection to electrical

earth. Class II equipment, may not have

provision for connection of exposed

metal work of the equipment to a

protective/earth conductor. Ex Mobile

chargers, Hand Dryers, TV etc.

4.2 Voltage Exposure

(Ref: IS 3043-2018, para 12.0.2)

When there is unintentional contact between an energized electric conductor (live

conductor) and the metal frame (body) or enclosure/ structure, the frame or structure

tends to become energized to the same voltage level as exists on the energized conductor.

In such cases, to avoid the dangerous voltage on exposed metal frame (body) the

equipment earthing conductor provides a low impedance path from the metal frame to the

zero potential earth junction (MET).

Impedance/Resistance of Protective conductor from point of installation (Equipment) to

MET should be sufficiently low enough to avoid creation of dangerous voltage on

equipment.

4.2.1 Avoidance of Thermal Distress

The earthing conductor also functions to carry the full earth fault current without

excessively raising the temperature of the earthing conductor or causing the expulsion of

arcs and sparks that could initiate a fire or explosion.

4.2.2 Preservation of System Performance

The earthing conductor must return the earth fault current back to the source without

introducing additional resistance/impedance to such an extent that would affect the

operation of OCPD.

4.2.3 Earthing &Protective Conductor

Earthing Conductor: - A protective conductor connecting the main earthing terminal (or

the equipotential bonding conductor of an installation when there is no earth bus) to an

earth electrode or to other means of earthing.(Ref: IS 3043-2018, para 3.13)

In general earthing conductor is the conductor connecting the MET to electrode. The

earth conductor designated as ‘E’ shall not be coloured i.e. it shall have the original

colour of bare conductor.(Ref: IS 3043-2018, para 11.2)

No switching device shall be inserted in the protective conductor, but

joints which can be disconnected for test purposes by use of a tool may

be provided.

Class II Equipment (Ref IS 9049:1980)

Equipment in which protection against

electric shock does not rely on basic

insulation only but in which additional

safety precautions such as double insulation

or reinforced insulation are provided, there

being no provision for protective earthing or

reliance upon installation conditions.



.Protective conductor

(Ref: IS 3043-2018, para 3.26)

Protective Conductor — A conductor used as a measure of protection against electric

shock and intended for connecting any of the following parts:

a) Exposed conductive parts,

b) Extraneous conductive parts,

c) Main earthing terminal, and

d) Earthed point of the source or an artificial neutral.

The protective conductor designated as ‘PE’ shall have bi-colour combination, green

and yellow (green/ yellow), for identification as protective conductor and for no other

purpose. (Ref: IS 3043-2018, para 11.2)

Figure 7: Marking of Earth Conductors

Bare conductors or bus bars, used as protective conductors, shall be coloured by equally

broad green and yellow stripes, each 15 mm up to 100 mm wide, close together, either

throughout the length of each conductor or in each compartment or unit or at each

accessible position. If adhesive tape is used, only bicoloured tape shall be applied.

For insulated conductors, the combination of the colours, green and yellow, shall be such

that, on any 15 mm length of insulated conductor, one of these colours covers 30-70%of

the surface of the conductor & other colour cover the remaining surface.

5.0 SYSTEMEARTHING (Ref: IS 3043:2018, para 11.0)

Earthing associated with current-carrying conductor which is normally essential to the

security of the system is known as System Earthing.(Ref IS 3043:2018,foreword para 6)

System earthing of low voltage installation is carried out to limit the potential of current

carrying conductor forming part of the electrical system with respect to general mass of

earth. Earthing of system is designed primarily to preserve the security of the system by

ensuring that the potential on each conductor is restricted to such a value as is consistent

with the level of insulation applied.

The basic objective of system earthing is to ensure efficient and fast operation of

protective device in the case of earth faults. The system earth fault loop impedance

should be such that, when any earth fault occurs, the protective device (circuit-breakers

or fuses etc.) will operate to isolate the faulty main or equipment.



In system earthing only one point (i.e. Neutral in TN &TT systems) is earthed to prevent

the passage of current through the earth under normal conditions and thus to avoid the

accompanying risks of electrolysis and interference with communication circuits. (In

case more than one point is earthed it will lead to creation of local loops in the system

there by flowing of circulating current which may cause electromagnetic induction).

6.0 TYPES OF SYSTEM EARTHING

Any system comprises mainly two portions, first the source of energy (transformer/ DG)

and second is installation. The source includes supply cables/ conductor connecting to

the installation. The supply system is generally denoted by two letters as shown below:

Figure 8: Indicative supply system based on system earthing

Description of letters:

The first letter refers to the relationship of the power system (source) to earth.

T ‘Direct connection of one point to earth’.

I ‘All the live parts isolated from earth, or one point connected to earth though a high

impedance’.

Second letter refers to the relationship of the exposed-conductive-parts of the installation

to earth’.

T ‘Direct electrical connection of exposed conductive parts to earth, independently of the

earthing any point of the power system’.

N ‘Direct electrical connection of exposed conductive parts to the earthed point of the

power system’. (in ac systems, the earthed point of the power system is normally the

neutral point or, if a neutral point is not available, a line conductor).

Subsequent letters if any,

S Separate neutral and protective earth functions provided by separate conductors i.e.PE &

N conductor.

C Combined neutral and protective earth functions provided by same conductor i.e.

PEN/CNE conductor.

The letters used in European Standards are derived from the French language:

T - Terre which represents Earth

N – Neutre which represents Neutral

S – Separe which represents Separate

C – Combine which represents Combined

I – Isole which represents Insulated

Electrical supply systems are further classified as TN system, TT system and IT system

based on system earthing. They are as given under.

Source

Side

Installation

Side

T N



6.1 TN System

TN system has one or more points of the source of energy(preferably Neutral point of

Transformer or DG)directly earthed at source end. The exposed and extraneous

conductive parts of the consumer installation are connected by means of protective

conductors to the earthed point(s) of the source. In this way, there is a metallic return

path for earth fault currents to flow from the installation to the earthed point(s) of the

source. TN systems are further sub-divided into TN-S,TN-C and TN-C-S systems.

6.1.1 TN-S System

In the system where separate conductors each for neutral and protective functions

are provided throughout the system is called as TN-S system (as shown blow in

figure).

Figure 9: TN-S System

6.1.1.1 For 230 V single phase domestic/commercial supply

(Ref: Fig. 12 of IS 3043-2018).

Figure 10: TN-S System for 230V Single Phase

Source TN-S SYSTEM (230 V SINGLE PHASE)R

Y

B

N

PE

EXPOSEDCONDUCTIVE PART

CONSUMERINSTALLATION

EQUIPMENT ININSTALLATION

SOURCE EARTH

Every supply neutral conductor of electrical network of voltage 650 V

and below is required to be connected with earth generally at the source

of voltage. (Ref- IS 3043-2018 para 4.1 Note 2)



The protective conductor (PE) is the metallic covering (armour or sheath of the

cable supplying the installation) or a separate conductor.

All exposed conductive parts of an installation are connected to this protective

conductor via main earthing terminal of the installation.

6.1.1.2 For 415 V three-phase domestic/commercial supply

415 V three phase domestic/ commercial supply having three phase and single

phase loads, all exposed conductive parts of the installation are connected to

protective conductor via the main earthing terminal of the installation.

It is necessary to install an independent earth electrode within the consumer’s

premises and same to be connected as shown in the figure given below (Ref: Fig.

13 of IS 3043-2018).

Figure 11: TN-S System for 415 V three-phase domestic commercial supply

6.1.2 TN-C System

In TN-C system, neutral and protective functions are combined in a single conductor

throughout the system (Ref: Fig. 14 of IS 3043-2018).

Figure 12: TN-C System

SINGLE PHASE LOAD

THREE PHASE LOAD EXPOSEDCONDUCTIVE PART

Source TN-S SYSTEM (415 V THREE PHASE)R

Y

B

N

PE

CONSUMERINSTALLATION

SOURCE EARTH

Installation EarthElectrode

EXPOSEDCONDUCTIVE PARTS

SINGLE PHASEINSTALLATION

COMBINED PE ANDN CONDUCTOR

(PEN)

Source TN-C SYSTEMR

Y

B

SOURCE EARTH

THREE PHASECONSUMER

Installation Earth

EXPOSEDCONDUCTIVE PARTS

Additional SourceEarth

TN-S system requires 5 conductors (three lines, one neutral and one

protective conductor), due to this, cost of installation is high as

compared to other systems.



All exposed conductive parts are connected to the PEN conductor.

For three phase consumer, local earth electrode has to be provided in addition.

TN-C systems are not suitable for modern installations with electronic equipment.

As the protective function and neutral function are combined in one conductor, in

case of PEN conductor breaks/ disconnects due to any reason (neutral breakage),

the exposed conductive parts in an installation becomes live, which may lead to

shock hazards.

Figure 13: Neutral breakage in TN-C System

6.1.3 TN-C-S System

In TN-C-S system, neutral and protective functions are combined in a single conductor

(PEN conductor)only in part of the system. Usually, the neutral and protective functions

are combined in a single conductor up-to consumer’s premises and separate at

installation side as shown in the figure below.

All exposed conductive parts of an installation are connected through protective

conductors. All the protective conductors/connections run separately inside consumer’s

premises and connected to main earthing terminal (MET)and then to the PEN conductor.

(Ref: Fig. 15 of IS 3043-2018).

Figure 14: TN-C-S System

Source TN-C-S SYSTEMR

Y

B

SOURCEEARTH

COMBINED PE AND NCONDUCTOR (PEN)

EXPOSEDCONDUCTIVE

PARTS

Earth Electrode

Installation Earth Electrode

Three Phase Consumer Installation



This system requires only four conductors in public distribution (i.e. up-to incoming

supply at consumer’s premises). This saves the cost of 5th

conductor in public

distribution.

The supply system PEN conductor is earthed at several points. Therefore, this type of

distribution is known also as Protective Multiple Earthing(PME) and the PEN

conductor is also referred to as the Combined Neutral and Earth (CNE)

conductor.

An earth electrode may be necessary at or near a consumer’s installation.

Multiple earthing of the PEN/CNE conductor ensures that if the conductor becomes

open circuit for any reason (probably cut or snapped), exposed-conductor parts

remain connected to earth, under such conditions the supply voltage between the

installation line and neutral conductor is substantially reduced and consumer will

experience unacceptable voltage variations.

Figure 15: Multiple Earthing of PEN Conductor in TN-C-S System

If PME is not provided to PEN conductor any breakage of it may cause dangerous

rise in potential of exposed-conductor parts and which may lead to shock hazards.

.

6.2 TT System

TT system has one or more points of the source of energy directly earthed and the

exposed and extraneous conductive parts of the installation are connected to a local earth

electrode. Source and installation earth electrodes are electrically independent. (Ref: Fig.

17 of IS 3043-2018).

As per statutory provisions for earthing mentioned in Cl. No. 4 of IS

3043-2018. The TN-C-S is the most common system adopted by

Electrical supply undertakings. TN-C-S system is also called as

Protective Multiple Earthed (PME) system



Figure 16: TT System

All exposed conductive parts of an installation are connected to an independent earth

electrode.

Single phase TT system is not present in India.

6.3 IT System

IT system has the source either unearthed or earthed through a high impedance and the

exposed conductive parts of the installation are connected to electrically independent

earth electrodes.(Ref: Fig. 18 of IS 3043-2018).

Figure 17: IT System

All exposed conductive parts of an installation are connected to an earth electrode.

The source is either connected to earth through deliberately introduced earthing

impedance or is isolated from earth.

IT system is used where continuity of supply is desired.

Y

B

N

Installation

Earth

Electrode

ConsumerInstallation

SOURCE EARTH

Exposed ConductiveParts

Exposed Conductive Parts

Three Phase LoadSingle

Phase

Load

SourceTT System

R

Source IT SystemR

Y

B

HIGH

IMPEDANCE

SOURCE

EARTHING

Consumer

Installation

Exposed Conductive

PartsExposed

Conductive Parts

Installation EarthElectrode

Installation Earth Electrode

Three Phase LoadSingle Phase

Load



7.0 EQUIPOTENTIAL BONDING

“Electrical connection putting various exposed conductive parts and extraneous

conductive parts at a substantially equal potential is termed as equipotential bonding”.

(ref: IS:3043-2018 para no. 3.15)

Equipotential is derived from Equal potential between two points. In a system,

connections of all non-current carrying (exposed conductive parts) parts of a current

using equipment like metal body of equipment, etc. & connections of all non-electrical

metallic items embedded in building structure(extraneous conductive parts) for ex

Reinforced steel in concrete, Metallic pipe lines, etc. to main earthing terminal in order to

bring all at same potential (Equipotential) in event of an earth fault is called

Equipotential bonding.

Figure 18: Equipotential bonding with all exposed/extraneous conductive parts

Structural steel (extraneous-conductive-part)

Metallic water pipe (extraneous-conductive-part)

Earth electrode (TT and IT system)

PE (TN systems)

1 Circuit protective conductor

2 Main protective bonding conductor

3 Earthing conductor

4 Supplementary protective bonding conductor (if required)

1,2,3,4 Protective conductors

Common equipotential bonding system has both protective equipotential bonding and

functional equipotential bonding.

44

2

2

PE3

Metallicwater pipes

1

11

Earth Electrode

StructuralSteelMET

ExposedConductive Parts

Protective Conductor

Supplementary Bonding Conductor

M

M

M



7.1 Protective Equipotential Bonding

(Ref: IS 732:2019 para 3.150, 4.2.11.3.1.2)

Equipotential bonding done for the purpose of safety is termed as “Protective

Equipotential bonding”. In this protection is achieved by automatic disconnection of

supply.

In each building the earthing conductor, the main earthing terminal and the following

conductive parts shall be connected to the protective equipotential bonding:

i. Metallic pipes supplying services into the building, for example, gas, water, fire

fighting, sewage etc.

ii. Structural extraneous conductive parts, if accessible in normal use, metallic central

heating and air-conditioning systems; and

iii. Metallic reinforcements of constructional reinforced concrete, if reasonably

practicable.

Figure 19: Equipotential Bonding with all exposed/extraneous conductive parts

Note :- Where such conductive parts originate outside the building, they shall be bonded

as close as practicable to their point of entry within the building.

7.2 Supplementary Equipotential Bonding

Supplementary Equipotential Bonding is also called as local equipotential bonding. This is

done to achieve high degree of safety against electric shock, Supplementary equipotential

bonding is considered as an addition to fault protection. This will also reduce the voltages

occurring in the event of an earth fault still further.

During design of an electrical system if the fault loop impedance which is required for

automatic disconnection of supply is not achieved through equipotential bonding, then it is

necessary to provide supplementary equipotential bonding.(Ref: IS 3043: 2018 para

11.1.3)

Steel Structure work

Load

Line

Neutral

Equip

men

t

MainProtectiveEquipotentialBonding

Metal pipe (eg. Fire fighting system)

Metal pipe (eg. Sewage system)

Metal pipe (HVAC/ ducting etc.)

Exposed conductive parts



Figure 20: Equipotential Bonding with Supplementary Bonding

7.3 Main Earthing Terminal (MET)

“The terminal or bar (which is the equipotential bonding conductor) provided for the

connection of protective conductors and the conductors of functional earthing, if any, to

the means of earthing. (Ref: IS 3043:2018, para no. 3.21)

The function of MET is to provide a reference point for the installation, it

consists of a terminal or bar provided for the connection of protective conductors and

conductors for functional earthing.

MET is connected to earth but it remains rarely at zero potential because of the

potential difference caused by leakage and other current flowing to earth.

Figure 21: Main Earthing Terminal (MET)

SupplementaryBonding (if necessary)

MainProtectiveEquipotentialBonding

MET(SUB)

Metal pipe (eg. Fire fighting system)

Metal pipe (eg. Sewage system)

Metal pipe (HVAC/ ducting etc.)


Load

Steel Structure work

Line

Neutral

Equip

men

t



7.4 Typical Schematic of Earthing and Protective Conductors

A typical schematic of earthing and protective conductors is shown in given below figure.

In the installation, all protective conductors of exposed conductive parts are connected to

main earthing terminal at distribution boards. Further the METs of all distribution boards

are connected to main MET. All extraneous conductive parts, incoming metallic service

pipes, are also connected to main MET. Further this main MET is connected to earth

electrodes by Earth conductor or PEN conductor as per design of the supply system.

Figure 22: Earthing Arrangements and Protective Conductors

Distribution Board

Main Earthing

Terminal(s)

(MET)

Current Using

Equipment

Earthing Conductor

Protective Conductor

Exposed

conductive

parts

Exposed

conductive

parts

No switching device shall be inserted in the protective conductor, but

joints which can be disconnected for test purposes by use of a tool may

be provided.

All connections shall be carefully made on MET if they are poorly

made or inadequate for the purpose for which they are intended, loss

of life or serious personal injury may result.

In the case of TT and IT system, the MET should be earthed by an

independent earth electrode only.

During designing of a system, earthing conductors are also protective

conductors and should be sized in the same way as other protective

conductors.



7.5 Extraneous Conductive Parts

(Ref IS 3043:2018 para 23.2.3 )

The extraneous conductive parts that are required to be bonded to the main earthing

terminal of the installation (or to the earth electrode of the installation) include:

a) Gas pipes,

b) Other service pipes and ducting,

c) Risers and pipes of fire protection equipment,

d) Exposed metallic parts of the building structure, and

e) Lightening conductors.

7.6 Exposed Conductive Parts

(Ref IS 3043:2018 para 23.2.4)

Exposed conductive parts that are required to be connected by means of protective

conductors to the main earthing terminal of the installation are as follows:

a) All metal structure associated with wiring system (other than current-carrying parts)

including cable metal sheaths and armour, conduit, ducting, trunking, boxes and

catenary wires.

b) The exposed metal structure of all Class I equipment fixed and portable current-

using equipment. All fixed wiring accessories should incorporate an earthing

terminal that is connected to the main earthing terminal by means of the protective

conductors of the circuits concerned.

c) The exposed metal structure of transformers used in the installation other than those

that are an integral part of equipment.

7.6.1 Exposed conductive parts not required to be earthed

Exposed conductive parts that (because of their small dimensions or disposition) cannot

be gripped or contacted by a major surface of the human body (that is, a human body

surface not exceeding 50 mm x 50 mm) need not be earthed if the connection of those

parts to a protective conductor cannot readily be made and reliably maintained.

Typical examples of such parts are:-

a) Screws and nameplate, cable clips and lamp caps.

b) Fixing screws for non-metallic accessories need not be earthed provided there is no

appreciable risk of the screws coming into contact with live parts.

Other exposed conductive parts not required to be earthed

c) Overhead line insulator brackets and metal parts connected to them if such parts are

not within arm’s reach.

d) Short lengths of metal conduit or other metal enclosures used give mechanical

protection to equipment of Class II or equivalent construction.



Direct Contact

Protection against direct contact (Basic protection)

(Ref: IS 732-2019 para 4.1.2.1

Protection against direct contact is also termed as basic protection in case of low-

voltage installations, systems and equipment. This type of protection is provided

against dangers that may arise from direct contact and can be achieved by one of the

following methods:

a) Preventing a current from passing through the body of any person or any

livestock; and

b) Limiting the current which can pass through a body to a non-hazardous value.

Measures for Protection against direct contact:

(Ref- IS732:2019 Annex A&B)

a) By provision of insulation on live parts: Live parts shall be completely covered

with insulation which can only be removed by destruction.

b) By provision of Barriers or enclosures: Live parts shall be inside enclosures or

behind barriers.

8.0 EARTH FAULT PROTECTION IN LV INSTALLATIONS

The occurrence of an earth fault in an installation creates two possible hazards. Firstly,

voltages appear between exposed conductive parts and extraneous conductive parts, and

if these parts are simultaneously accessible, these voltages (potential difference)

constitute a shock hazard, this condition being known as Indirect contact.

Figure 23: Indirect Contact in case of earth fault

Secondly, the fault current that flows in the phase and protective conductors of the circuit

feeding the faulty equipment may be of such a magnitude as to cause an excessive

temperature rise in those conductors, thereby creating a fire hazard.

Contact of persons or livestock with live parts of electric

supply/ installation/ equipment which may result in electric

shock is termed as direct contact.



8.1 Protection against Indirect Contact

Protection against indirect contact is achieved by the adoption of one of the following

protective measures:

1) Safety extra low voltage(IS 9409:1980 para no 2.6)

A voltage which does not exceed 50 V ac rms between conductors or between any

conductor and earth in a circuit which is isolated from the supply mains by means of

a safety isolating transformer or converter with separate windings.

2) The use of Class II equipment or by equivalent insulation

(The detail of classes of equipment are explained in annexure)

3) A non-conducting location(IS 732:2019 Para C-1)

The non-conducting location is a special arrangement where there is no earthing or

protective system provided because there is nothing which needs to be earthed. In this

non-conducting location all exposed conductive parts are so arranged that it is

impossible to touch two of them, or an exposed conducting part and an extraneous

conductive part, at the same time. This measure can only be applied in a dry

location.

Figure 24: A Non Conductive Location

4) Earth free local equipotential bonding (IS 732:2019 Para C-2)

Earth-free local equipotential bonding is provided to prevent the appearance of a

dangerous touch voltage by connecting equipotential bonding conductors with all

simultaneously accessible exposed conductive parts and extraneous conductive parts

without electrical connection to the earth directly and indirectly (i.e through exposed

conductiveparts and extraneous-conductive-parts)

5) Electrical separation

Electrical separation is a protective measure in which, basic protection is provided by

basic insulation of live parts or by barriers/enclosures and fault protection is provided

by simple separation of the separated circuit from other circuits and from earth.

6) Earthed equipotential bonding and automatic disconnection of the supply

In order to achieve a high degree of protection against both shock and fire hazards the

earthed equipotential bonding and automatic disconnection of the supply is used.

Further it is explained in detailed below.



8.2 Earthed Equipotential Bonding and Automatic Disconnection of the Supply

The two aims of this protective measure are to:

a) Ensure that when an earth fault occurs, the voltages appearing between exposed

conductive parts and extraneous conductive parts in the location served by the

installation concerned are minimized; and

b) Ensure rapid disconnection of the circuit in which that earth fault occurs.

In order to meet (a), a zone is created by connecting all extraneous & exposed conductive

parts by means of equipotential bonding conductors to the MET and earth electrode of

the installation is called equipotential zone as explained earlier in para 7.0 in this booklet.

This does not mean that voltages cannot exist between conductive parts in that zone

when an earth fault occurs a voltage will appear between the exposed conductive parts

and extraneous conductive parts in the location served by the installation concerned, but

the application of protective equipotential bonding minimizes these touch voltage reaches

a harmful value.

An installation may consist of a number of zones; for instance, when an installation

supplies a number of buildings, equipotential bonding is necessary in each building so

that each constitutes a zone having a reference point to which the exposed conductive

parts of the circuits and current-using equipment in that building are connected. (Ref: IS

3043:2018 para 23.2.2.1)

The second aim of this protective measure is to disconnect the supply to the faulty

equipment within the prescribed time by limiting the upper value of the earth fault loop

impedance of each circuit. On the occurrence of an earth fault (assumed to be of

negligible impedance), disconnection will occur before the prospective touch voltage

reaches a harmful value.

The choice of protective device used to give disconnection is influenced by the type of

system of which the installation is part, because either:

1) The earth fault loop impedance has to be low enough to allow adequate earth

fault current flow in order to cause operation of an over current protective device

(for example, a fuse or MCB, MCCB) in a sufficiently short time.

2) Where it is not possible to achieve a low enough earth fault loop impedance,

disconnection may be initiated by fitting a Residual Current Device (RCD) of 30

mA rating. (Ref- IS 732:2019 para 4.2.11.1).

Protection against shock in case of a fault (protection against indirect contact) is provided

by automatic disconnection of supply and this is to prevent a touch voltage persisting for

such long time that may be dangerous.

Automatic disconnection of supply following an insulation fault relies on the association

of two conditions given below:

a) The existence of a conducting path (fault loop) to provide for circulation of fault

current (this depends on type of system earthing); and

b) The disconnection of this current by an appropriate device in a given time.

The determination of this time depends on various parameters, such as probability of

fault, probability of a person touching the equipment during the fault and the touch

voltage to which a person might thereby be subjected.



8.2.1 Disconnecting Times for Different Touch Voltages

Limits of touch voltage are based on studies on the effects of current on human body

(Ref IS/IEC 60479:2005). Table 2shows the values disconnection time limits (t) for

different touch voltages for two most common conditions (dry and wet conditions) for

safety of human beings. If the system has to protect the human, it should trip with in the

stipulated time limits. During the designing of system, the selection of protective devices

can be done based on the tripping time w.r.t the prospective fault current. This fault

current can be calculated with the help of System voltage and Fault loop impedance.

Table 2: Disconnecting Times for Different Touch Voltages

(Ref- IS 3043:2018, Table 13)

Prospective Touch

Voltage Uc

Condition 1

Dry or moist locations, dry skin

and significant floor resistance

Condition 2

Wet locations, wet skin and

low floor resistance

Z1 I t Z2 I t

(V) () (mA) (s) () (mA) (s)

(1) (2) (3) (4) (5) (6) (7)

25 -- -- -- 075 23 5

50 1725 29 5 925 54 0.47

75 1625 46 0.60 825 91 0.30

90 1600 56 0.45 780 115 0.25

110 1535 72 0.36 730 151 0.18

150 1475 102 0.27 660 227 0.10

220 1375 160 0.17 575 383 0.035

280 1370 204 0.12 570 491 0.020

350 1365 256 0.08 565 620 --

500 1360 368 0.04 560 893 --

Z1- Total Impedance of human body in Dry condition (IS 60479:2005 part 1)

Z2- Total Impedance of human body in wet condition (IS 60479:2005 part 1)

I – Current that will flow through the human body

t- Disconnecting time limit which will not result in harmful physiological effects on Human

body.

8.3 Requirement for Automatic disconnection in TN System

(Ref IS 3043:2018 para 23.0.7)

In TN system the characteristics of the protective devices and the cross-sectional area of

conductors shall be so chosen that if a fault of negligible impedance occurs anywhere

between a phase conductor and a protective conductor or exposed conductive part,

automatic disconnection of the supply will occur within the minimum possible safe time.

The time of operation would depend on the magnitude of the contact potential.

This requirement is met if:

ZsxIa≤Uo……………….(1)

where

Zs = fault loop impedance,

Ia = current ensuring the automatic operation of disconnecting device, and

Uo = conventional voltage limits.

Zs may be calculated or measured.



The duration of Ia permitted depends on the prospective touch voltage (please refer

table 2 for prospective touch voltage and corresponding timings).Higher touch

voltages should be cleared in shorter times.

If this condition cannot be fulfilled, supplementary bonding may be necessary.

8.4 Requirement for Automatic disconnection in TT System

(Ref IS 3043:2018 para 23.0.8)

In TT system the characteristics of the protective devices and the cross-sectional area of

conductors shall be so chosen that if a fault of negligible resistance occurs any where

between a phase conductor and a protective conductor or exposed conductive parts,

automatic disconnection of the supply will occur within the minimum possible safe time.

The time of operation would depend on the magnitude of the contact potential.

This requirement is met if:

RAxIa≤ Uc………………..(2)

where

RA = resistance of the earthed system for exposed conductive parts,

Ia = operating currents of the disconnecting series device or settings of shunt

relays, and

Uc = conventional voltage limit

8.5 Requirement for Automatic disconnection in IT System

(Ref IS 3043:2018 para 23.0.9)

The impedance of the power system earth shall be such that on the occurrence of a single

fault to exposed conductive parts or to earth, the fault current is of low value.

Disconnection of the supply is not essential on the occurrence of the first fault. Protective

measures must, however, prevent danger on the occurrence of two simultaneous faults

involving different live conductors.

The following condition shall be fulfilled:

RAx Id≤ Uc………………(3)

where

RA = resistance of the earthed system for exposed conductive parts,

Id = operating currents of the disconnecting series device, and

Uc = conventional voltage limit.

9.0 EARTH FAULT LOOP IMPEDANCE

Impedance/resistance encountered by fault current under earth fault condition is called

“Earth fault loop impedance” i.e. the impedance of the earth fault current loop (phase

to earth loop) starting and ending at the point of earth fault.(Ref: IS:3043:2018 para

3.10). This impedance is denoted by the symbol Zs.

The earth fault loop comprises the following, starting from the point of fault (Ref:

IS:732:2019 para 3.64)

a) The circuit protective conductor,

b) The consumer’s earthing terminal and earthing conductor, and for TN systems, the

metallic return path,

c) For TT and IT systems, the earth return path,

d) The path through the earth neutral point of the transformer,



e) The transformer winding, and

f) The line conductor from the transformer tothe point of fault.

9.1 Fault Loop in TN-S System

In TN-S system, the earth fault loop impedance comprises the impedance starting from

the point of fault, protective conductor up to source, neutral point of the transformer,

transformer winding (involved in faulty circuit) and phase (line) conductor from the

transformer to the point of fault. A sample schematic is shown below:

Figure 25: Fault loop impedance circuit in TN-S system

9.2 Fault Loop in TN-C System

In TN-C system, the earth fault loop impedance comprises the impedance starting from

the point of fault, protective& neutral conductor (PEN) up to source, neutral point of the

transformer, transformer winding (involved in faulty circuit) and phase (line) conductor

from the transformer to the point of fault. A sample schematic is shown below:

Figure 26: Fault loop impedance circuit in TN-C system

Transformer Neutral

R

Y

B

N

Transformer BodyOPCD

EarthFault

Consumer Installation

Source

Fault Loop in TN-S

Load


PE

SourceEarth


Source

Fault Loop in TN-C

SourceEarth

Load


Transformer Neutral

R

Y

B

PEN

Transformer BodyOPCD

EarthFault

Whenever fault occurs the current will flow through this fault loop and

protection system will be designed accordingly with prospective fault

current. This fault current can be calculated with the help of System

voltage and Fault loop impedance.



9.3 Fault Loop in TN-C-S System

In TN-C-S system, there will be two loops for flow of fault currents, first through the

metallic return path i.e PEN conductor & second through the earth (Soil). The magnitude

of fault current flowing through the each path will depend on its impedance. Practically

the major portion of fault current through the first path i.e PEN conductor. This fault loop

impedance in this path comprises the impedance starting from the point of fault,

protective conductor (PE) up to MET (Neutral link at main incoming supply),protective&

neutral conductor (PEN) up to source, neutral point of the transformer, transformer

winding (involved in faulty circuit) and phase (line) conductor from the transformer to

the point of fault. The other earth fault loop through installation earth electrode, soil back

to source earth electrode. A sample schematic is shown below:

Figure 27: Fault loop impedance circuit in TN-C-S system

9.4 For TT System

In TT system, the earth fault loop impedance comprises the impedance starting from the

point of fault, circuit protective conductor up to MET, earthing link to installation earth

electrode conductor, installation earth electrode, soil path resistance from installation

earth electrode to source earth electrode, source earth electrode conductor from source

earth electrode to neutral point of the transformer, transformer winding (involved in

faulty circuit) and phase (line) conductor from the transformer to the point of fault. A

sample schematic is shown below:

Figure 28: Fault loop impedance circuit in TT system

Transformer Neutral

R

Y

Fault Loop in TN-C-S

OPCD

Load

Earth

Fault



B

PEN

Transformer Body

Source

EarthSoil Consumer Earth

Transformer Neutral N

Transformer Body

SourceEarth

R

Y

B

Fault Loop in TT

OPCD

Earth

Fault



Load

Soil Consumer Earth



10.0 EARTHING OF INSTALLATIONS

10.1 Earthing in Standby Generating Plants (Ref - IS 3043:2018 para 29)

In any low voltage installation where, regular supply is coming from state electricity

board and standby supply is being provided by Diesel Generator/ Alternator (DG/ DA)

set, earthing of standby generator set is necessary to protect against indirect contact.

The protective earthing conductors are connected as shown in below figure. Generating

set metal body, alternator metal frame, neutral point of alternator, metallic cable sheaths

and armouring, and all exposed conductive parts of installation (load) are connected by

earthing conductors to main earthing terminal (bar) and MET is further connected to

earth (normally at one point only). Neutral earthing of DG, earth link of electricity board

incoming supply are also connected to MET by protective conductors as shown below:

Figure 29: Single Low Voltage Standby Generator (Without paralleling Facility)

Four-pole changeover switching between the mains and standby, supplies should be

used to provide isolation of the generator and electricity board neutral earths. (Ref -

IS 3043:2018 para 29.2.2.2)

10.1.1 Unearthed Generators (Rating Below 10 kW) Supplying a Fixed Installation

(Ref - IS 3043:2018 para 28.2)

Where an unearthed generator is to supply a fixed installation, for the purpose of

protection against indirect contact in case of any earth fault occurs following measure are

recommended which will initiate automatic disconnection of supply and isolation of faulty

equipment/circuit.

a. One pole of a single phase generator should be connected to the installation main

earthing terminal.

Earth fault loop impedance tester is used for measurement of earth

Fault loop impedance. It is very useful for testing of electrical

installations for ensuring the safety.



b. The installation should be protected by RCDs.

c. The main earthing terminal should be connected to an earth electrode. The

resistance of electrode to earth should sufficiently low to operate the RCDs.

d. All exposed-conductive parts and all extraneous-conductive-parts in the

installation are to be connected to the main earthing terminal.

Figure 30: Small low voltage generator supplying a fixed installation

10.2 Unearthed Generators Supplying a Mobile or Transportable Unit

(Ref - IS 3043:2018 para 28.3)

Where an unearthed generator is to supply a mobile installation, for the purpose of

protection against indirect contact in case of any earth fault occurs following measure are

recommended which will initiate automatic disconnection of supply and isolation of faulty

equipment/circuit.

a. One pole of a single phase generator should be connected to the installation main

earthing terminal.

b. All exposed-conductive parts and all extraneous-conductive-parts are to be connected

to the main earthing terminal.

c. The installation should be protected by RCDs.

d. If the unit supplies equipment or socket outlets outside the unit these circuits should

be protected by RCDs with a rated residual operating current not exceeding 30 mA

and an operating time not exceeding 40 ms at a residual current of 5 times rated

current I.

If practicable, an earth electrode should be connected to the main earthing terminal of the unit with a resistance to earth sufficiently low to operate the RCDs.

RCD

Unearthed Generator

Isolator

Main Earth Terminal

PE

Fixed Electrical Installation

Earth ElectrodeExposed Conductive Parts

Load

Exposed Conductive Parts



Figure 31: Small unearthed generator supplying a mobile or transportable unit

10.3 Earthing in Building

(Ref - IS 732:2019 Annex FF)

In any building installation, using LT supply for lighting, air conditioning, heating etc.,

the entire installation metallic components shall be connected to each other for creating

equipotential zone to prevent personnel from shock hazard in case of indirect contact

during any earth fault.

All exposed conductive parts of all current using equipment shall be connected to MET

via protective earthing conductors through sub distribution boards and main distribution

board. All metallic water and waste pipe lines in the bathrooms/ toilet shall be connected

through supplementary equipotential bonding terminal (SEBT) by protective bonding

conductors and this SEBT is to be connected to MET by protective earthing conductors.

All metallic fittings to heating and air-conditioning system (ducts, grills etc.) shall also

be connected to MET by protective bonding conductors. Other metallic pipe lines

coming to installation and other metallic structures within the building shall be connected

to MET by protective bonding conductors.

All protective conductors of sub distribution boards shall be connected to MET through

main distribution board. Main distribution board protective conductor may be connected

to PE/PEN conductor as per requirement of the system (in case of TN-S to PE or in case

of TN-C, TN-C-S to PEN conductor of supply system).

The MET shall be connected to Concrete-embedded foundation earth electrode or soil-

embedded foundation earth electrode by earthing conductor.

The down conductor of lightning protection system (LPS) if provided shall be connected

to Concrete-embedded foundation earth electrode or soil-embedded foundation earth

electrode as well as to earth electrode provided for LPS. An example of earthing

arrangements and protective conductors is shown below:

PE

Isolator

Unearthed GeneratorMobile Unit

RCD

Load

Exposed Conductive PartsExposed Conductive Parts

The RCD will not provide protection for faults on the generator side

of the RCD, and consequently precautions should be taken.



Figure 32: Example of Earthing Arrangements and Protective Conductors

Symbol Name Symbol Name

1 Protective earthing conductor (PE) T1 Earth electrode for LPS if

necessary.

1a Protective conductor, or PEN

conductor, if any, from supplying

network

PE PE terminal(s) in the main

distribution board

2 Protective bonding conductor for

connection to main earthing

terminal.

PE/PEN PE/ PEN terminal(s) in the main

distribution board

22

3

Lig

htn

ing P

rote

ctio

n S

yst

em

3

Soil embedded or Concrete embedded foundationEarth Electrode

InsulatingInsert

GroundSurface

Waterpipe

(Metal)

Wastewater pipe

(metal)

Gas pipe withinsulating insert(metal outside)

T1T1

Air Conditioning

Heating system

MetallicWater pipe

Waste waterpipe (metal)

Exposedconductive part

Exposedconductive part

Room

SEBT

Earthing Conductor

Ear

thin

g C

onduct

or

Earthing Conductor

1

1

1

1

Bathroom

PE

PE/PEN

1a

Sub DistributionBoard

Main EarthingTerminal

Main Distribution Board

Lig

htn

ing P

rote

ctio

n S

yst

em



3 Protective bonding conductor for

supplementary bonding

Extraneous-conductive part (Water pipe, metal

from outside, Waste water pipe, metal from

outside, Gas pipe with insulating insert, metal

from outside) 4 Down conductor of a lightning

protection system (LPS) if any.

5 Earthing conductor

11.0 EARTH ELECTRODES

“A conductor or group of conductors in intimate contact with and providing an

electrical connection to earth”.(Ref: IS3043:2018 para3.7)

11.1 Effect of Shape on Electrode Resistance

(Ref: IS3043:2018 para 14.1)

The greater part of the fall in potential occurs in the soil within a few feet of the

electrode surface, since it is here that the current density is highest. To obtain a low

overall resistance of the earth the current density should be as low as possible in the

medium adjacent to the electrode& it should be so designed as to cause the current

density to decrease rapidly with distance from the electrode.

To achieve the above requirement the dimensions (i.e Length, width & thickness) of

earth electrode is made large in one direction compared with other 2 directions (), thus

a pipe, rod or strip has a much lower resistance than a plate of equal surface area.

However, resistance is not inversely proportional to the surface area of the electrode.

Figure 33: Sphere of Influence Earth Electrode

11.2 Multiple Earth Electrodes

In this system more than one electrode is driven into the ground and connected in

parallel to lower the resistance. Each earth electrode has its own sphere of influence and

for additional electrodes to be effective the spacing of additional rods needs to be at

least equal to the depth of the driven rod. Without proper spacing of earth electrodes,

the spheres of influence will intersect and the lowering of the resistance will be minimal

and of little value.

Figure 34: Spheres of Multiple Earth Electrodes



.

11.3 Common Types of Earth Electrodes

11.3.1 Plate electrode

Figure 35: Copper Plate Electrode

According to Cl. 14.2.1 of IS3043-2018, the approximate resistance to earth of a plate

can be calculated from:

where

= resistivity of the soil in -m (assumed uniform), and

A = area of both sides of the plate (in m2).

Plate electrodes shall be of the size at least 60 cm × 60 cm. Plates are generally of cast

iron not less than 12 mm thick and preferably ribbed. The earth connection should be

joined to the plate at not less than two separate points. Plate electrodes, when made of

GI or steel, shall be not less than 6·3 mm in thickness.Plate electrodes of Cu shall be

not less than 3·15 mm in thickness. Plate electrodes shall be buried such that its top

edge is at a depth not less than 1·5 m from the surface of the ground.

11.3.2 Pipes or Rods electrode

According to Cl. 14.2.2 of IS3043-2018, the resistance of a pipe or rod electrode is

given by:

Where,

ρ = resistivity of the soil, in Ω-m (assumed uniform).

l = length of rod or pipe, in cm,

d = diameter of rod or pipe, in cm,

The use of coke breeze as an infill is not recommended as it may result

in rapid corrosion not only of the electrode itself but also of cable

sheaths, etc, to which it may be bonded.

It is recommended that a drawing showing the main earth connection

and earth electrodes be prepared for each installation.



The earth resistance diminishes rapidly with the first few feet of driving, but less so at

depths greater than 2 to 3 m in soil of uniform resistivity. If required to achieve less

resistance a number of rods or pipes may be connected in parallel than to increase the

length of earth electrode.

Figure 36: Pipe Electrode

Figure 37: Copper Bonded Steel rod Electrode

11.3.2.1 Erection of pipe Earth electrode

Construction of pipe electrode along with earth pit shall be as per figure shown

below (Fig 25 of IS 3043:2018) all dimensions are in millimetre. The earth fill shall

be of homogenous layers of common earth soil (In Earlier version of IS 3043:1987

with amendments the earth fill was a homogenous layer of coke/charcoal, salt).

Where soil resistivity is high, earth resistivity enhancement material may be used.

Bentonite or similar earth enhancing material may be advantageous in rocky terrain.

Bentonite packing will increase the contact efficiency with the general mass of

ground.

Mutual separation between multiple earth electrodes of Length (L)

should be between L to 2L. In case of mutual separation is not

possible between L to 2L, a substantial gain is effected even at 2 m

separation.

Copper clad mild steel rods can be used in place of normal mild steel

rods where the soil is corrosive.



Figure 38: Typical arrangement of pipe electrode (Ref fig 25 of IS 3043:2018)

WIRE MESH

SEEDETAIL

C.I.COVER HINGED

TO C.I. FRAME

DIA 65 PVC CONDUIT

EMBEDDED TO

GROUNDING

CONDUCTOR

HOMOGENOUS LAYER OF

COMMON EARTH/ EARTH

RESISTIVITY ENHANCINGMATERIAL WHERE

REQUIREDAFTER FIXING SURFACE

SHOULD BE COVERED WITH

BITUMIN OR GREASE

C.I.PIPE 100 ID

13 THICK

50 X 6

G.I.STRIP

DIA 10

BOLT AND

NUTS 50 X 3 G.I.

STRIP

400125 125

100

250

150 1250

3000

250

250

150 150

50 75

25

DETAIL A

AS PER IS 3043: 1987

AMENDMENT NO.2, JAN

2010: A HOMOGENOUS

LAYER OF COKE/

CHARCOAL AND SALT.

In case of rocky area where it is difficult to drive the electrode upto

required depth or high resistivity soil occur at relatively small depths,

considerable advantage may result from driving rods at an angle of

about 30° to the horizontal, thus increasing the installed length of

electrode for a given depth.



11.3.3 Artificial treatment of soil

(Ref – IS 3043:2018 para 13.8)

In areas of high soil resistivity, and where it is difficult to achieve low resistance to

earth even after providing multiple earth electrodes, for such types of locations artificial

treatment of soil by adding earth enhancing material can be done.

Such installation requires constant monitoring and replacement of the additives as

migration and leaching of applied chemicals over a period of time reduces the

efficiency of the system. Chemical treatment of soil also has environmental effects and

should not be considered as a long term solution in order to meet a specified level of

resistance.

To reduce the soil resistivity of the soil immediately surrounding the earth electrode,

alternative earth enhancing material which are easily dissolvable in the moisture

(contained in the soil), and are highly conductive may be used. These alternative

substances are common salt (sodium chloride = NaCl), calcium chloride (CaCl2),

sodium carbonate (Na2CO3), copper sulphate (CuSO4), salt & soft coke, and salt &

charcoal in suitable proportions. These agents form a conducting electrolyte throughout

a wide region surrounding the earth electrode. In using artificial treatment, the possible

corrosive effect of the earth enhancing material salt on the driven rods and connections

should be considered.

11.3.3.1 Procedure for Artificial treatment

In general when artificial treatment of earth electrode pit is done, it is done by

dissolving salt in water and further pouring this solution inside the hollow earth

electrode pipe considering that the solution will disperse around the electrode through

the perforated earth electrode but this is not the correct practice.

As per IS 3043:2018 para 13.8.1 approximately 90 percent of the resistance between a

driven rod and earth lies within a radius of about 2 m from the rod. This should be kept

in mind when applying the agents for artificial treatment of soil. The correct & simplest

application is by excavating a shallow basin around the top of the rod, 1 m in diameter

and about 30 cm deep, and applying the artificial agent in this basin. The basin should

subsequently be filled several times with water, which should be allowed each time to

soak into the ground, thus carrying the artificial treatment, in electrolyte form, to

considerable depths and allowing the artificial agent to become diffused throughout the

greater part of the effective cylinder of earth surrounding the driven rod.

11.3.4 Strip or Conductor Electrodes

According to Cl. 14.2.3 of IS3043-2018, the resistance of a strip or conductor electrode

is given by:

where,

ρ = resistivity of the soil, in Ω-m (assumed uniform),

l = length of the Strip in cm, and

d = width (strip) or twice the diameter (conductors) in cm.



Strip or conductor electrode has special advantages where low resistivity soil is at

shallow depth and high resistivity soil below surface layers of low resistivity soil. In

such cases Plate/pipe/rod electrode will not give satisfactory results. As seen from the

graph below, the earth resistance of electrode reduces substantially upto certain length,

further increase in length of electrode will not have any significant reduction in earth

resistance.

Figure 39: Effect of Length of Strip or Conductor Electrodes In Calculated Resistance

For Soil Resistivity Of 100 ΩM (Assumed Uniform)

To reduce resistance further strip or conductor electrode may be connected in parallel or

they may radiate from a point. In case of parallel connections of two strips at a

separation of 2.4 m, the total resistance (Rt) will be less than 65% of the individual

resistance (R) (i.e Rt<0.65R)

11.4 Other Means of Earthing

11.4.1 Cable Sheaths

Where an extensive underground cable system is available, the sheath and armour form

a most effective earth-electrode. In the majority of cases, the resistance to earth of such

a system is less than 1 Ω.Cable sheaths are more commonly used to provide a metallic

path to the fault current returning to the neutral.

11.4.2 Structural Steelwork as Earth Electrodes

The resistance to earth of steel frames or reinforced concrete buildings will vary

considerably according to the type of soil and its moisture content, and the design of the

stanchion bases. For this reason, it is essential to measure the resistance to earth of any

structural steelwork that it is employing and at frequent intervals thereafter. These are

called as foundation earth electrodes, which is a recommended practice in modern

electro-technical standards.

The care should be taken in positioning of these electrodes so as to

avoid damages during civil or agriculture work/operations.



11.4.3 Reinforcement of Piles

At power stations and large substations, it is often possible to secure an effective earth-

electrode by making use of the reinforcement in concrete piles. The earth strap should

be bonded to a minimum of four piles and all the piles between the bonds should be

bonded together. Each set of four piles should be connected to the main earthing-strap

of the substation.

Figure 40: Earthing of Foundation Reinforcement

Top ring should be half the size of main vertical reinforcement rod and to be

welded to main reinforcement rods.

Earthing Pads should be welded to reinforcement rods and shall be connected to

main earth grid.

Inserts other than earthing pads may or may not be welded to reinforcement.

11.4.4 Water Pipes

All Metallic pipe systems of services like water service, flammable liquids or gases,

heating systems, etc. shall not be used as earth electrodes for protective purposes.

For existing installations in which a water pipe is used as a sole earth electrode; an

independent means of earthing should be provided at the first practicable opportunity.

11.5 Connections to Earth Electrodes

The materials used for making connections have to be compatible with the earth rod and

the copper earthing conductor so that galvanic corrosion is minimized. In all cases, the

connections have to be mechanically strong.

For large earthing installations, such as at major substations, it is common to make

provision for the testing of earth electrodes. This is achieved by connecting a group of

Water pipes shall not be used as consumer earth electrodes.



rod driven electrodes to the main earth grid through a bolted link adjacent to the

electrodes in a sunken concrete box. Simpler disconnecting arrangements (or none at

all) may be acceptable for small earthing installations.

The connection of an earthing conductor to an earth electrode shall be soundly made

and electrically satisfactory. Where a clamp is used, it shall not damage the electrode

or the earthing conductor.

A consumer’s electrical installation of voltage 250 V but not exceeding 650 V supplied

from TN (TN-S, TN-C-S, TN-C) distributor network should have a main earthing

terminal (MET)that is connected to the protective conductor of the source and via this

to earth electrode installed in the electricity distributor supply system.

12.0 EARTHING SYMBOLS (Ref- IEC 60417)

No.5017 Earth (ground): To identify an earth terminal in cases where neither

the symbol 5018 nor 5019 is explicitly stated.

No.5018 Functional Earth: To identify a noiseless (clean) earth terminal, of

a specially designed earthing system to avoid causing malfunction of the

equipment. (e.g. connection of sensitive electrical equipment of circuits

directly to the PE conductor or to a functional earthing conductor (FE), to

minimize common mode distribution).

No.5019 Protective Earth: To identify any terminal which is intended for

connection to an external conductor for protection against electrical shock in

case of a fault, or the terminal of a protective earth electrode. (e.g. protective

conductor connecting point to an electrical equipment).

No.5020 Frame or Chassis: To identify a frame or chassis terminal. (e.g. it is

a measure to enhance the immunity of the equipment against conducted and

radiated RF disturbance. The connection of sensitive electrical circuits to the

chassis).

No.5021 Frame or chassis Equipotentiality: To identify the terminals when

connected, bring the various parts of an equipment or of a system to the same

potential, not necessarily being the earth potential, e.g. for local Equipotential

bonding.

Aluminium or copper clad aluminium conductors should not be used

for final connection to earth electrode

As far as possible, all earth connections shall be visible for inspection.



No 5173 Signal low terminal: To indicate the signal terminal the potential of

which is closest to the earth or chassis potential.

13.0 VOLTAGE GRADIENT AROUND EARTH ELECTRODES

During earth fault conditions, the earth electrode is raised to a potential with respect to

the general mass of the earth. This potential can be calculated by the magnitude of fault

current and the earth resistance of the electrode (Potential rise = Fault current × Earth

electrode resistance). This results in the existence of voltages in the soil around the

electrode that may be dangerous to livestock.

An effective remedy is to earth the neutral conductor at some point(PME) on the

system inaccessible to animals rather than earthing the neutral at the transformer itself.

Alternatively, an effective method is for pipe or rod electrodes to be buried with their

tops below the surface of the soil and connection made to them by means of insulated

leads. The maximum voltage gradient over a span of 2 m adjacent (for a 25 mm diameter

pipe electrode) is reduced from 85 percent of the total electrode potential at ground level

to 20 and 5 percent when the top of the electrode is buried below 0·3 m and 1·0 m

respectively.

Earth electrodes, other than those used for the earthing of the fence itself, should not be

installed in proximity to a metal fence, to avoid the possibility of the fence becoming live

and thus dangerous at points remote from the substation or alternatively giving rise to

danger within the resistance area of the electrode by introducing a good connection with

the general mass of the earth.

14.0 CURRENT DENSITY AT THE SURFACE OF EARTH ELECTRODE

An earth electrode should be designed to have a current carrying capacity adequate for

the system of which it forms a part, that is, it should be capable of dissipating energy

without failure. Failure of earth electrode is mainly due to excessive temperature rise at

the surface of the electrode and is thus a function of current density and duration as well

as electrical and thermal properties of the soil. In general, soils have a negative

temperature coefficient of resistance so that sustained current loading results in an initial

decrease in electrode resistance and a consequent rise in the earth fault current for a

given applied voltage.

Based on experimental studies on the subject by experts at the international level has led

to the following conclusions:

a) Long-duration loading due to normal unbalance of the system will not cause failure

of earth-electrodes provided that the current density at the electrode surface does not

exceed 40A/m2. Limitation to values below this would generally be imposed by the

necessity to secure a low-resistance earth.

b) Time to failure on short-time overload is inversely proportional to the specific

loading, which is given by the i2, where i is the current density at the electrode

surface. For the soils investigated, the maximum permissible current density, i is

given by:



i

where

t = duration of the earth fault (in s); and

ρ = resistivity of the soil (in Ω.m).

Experience indicates that this formula is appropriate for plate electrodes.

15.0 BASICS OF ELECTRIC CIRCUIT

Let us revise some basics of electrical circuit which will be helpful for understanding

better.

15.1 Simple Open and Closed Circuit

Current leaves the source and returns to the source. That is called a circuit. If you take a

battery (power supply source), the circuit is complete ONLY if you have one part of the

circuit connected to the positive and the other side to the negative.

Figure 41: Incomplete circuit or open circuit, no current will flow in above condition.

Figure 42: Complete circuit or close circuit, current will flow in this condition according

to the source voltage and load resistance.

15.2 Open and Closed Circuit with Earth

There is NO circuit if you connect one side to the positive and the other side to ground.

Try it. Take a battery and connect positive to the one terminal of the light bulb and

connect the other terminal to earth. It will not go on. (Do this with a battery, not a 220 V

power supply).



Figure 43: Incomplete circuit or open circuit, no current will flow in above condition.

Figure 44: Complete circuit or close circuit, current will flow in this condition according

to the source voltage and circuit resistance.

16.0 ELECTRIC SHOCK (Cl. No. 7 of IS 3043-2018)

16.1 Range of Tolerable Current

Effects of an electric current passing through the vital parts of a human body depend on

the duration, magnitude, and frequency. The most dangerous consequence of such an

exposure is a heart condition known as ventricular fibrillation, resulting in immediate

arrest of blood circulation.

A circuit connected to the earthing arrangement (protective equipotential bonding) will

protect us from an electrical shock if the supply is disconnected within the recommended

time. Connection to earth electrode (plate/pipe/rod) will not protect us from electric

shocks. The ONLY way to prevent or stop an electric shock is to turn off the circuit

before it creates a shock.

Current does not go to Earth. Current is always trying to make it back

to the source.



16.2 Effect of Frequency

Humans are very vulnerable to the effects of electric current at frequencies of 50 Hz.

Currents of approximately 100 mA can be lethal. Research indicates that the human body

can tolerate a slightly higher 25 Hz current and approximately five times higher direct

current.

16.3 Effect of Magnitude and Duration of Current Passing through Body

Table 3 : Effect of Magnitude and Duration of Current Passing through Body

S.no Current Range Repercussion on Body

1 1 mA Threshold of perception; that is, the current magnitude at which a

person is just able to detect a slight tingling sensation in his

hands or fingertips.

2 1 mA to 6 mA Let-go currents, though unpleasant to sustain, generally do not

impair the ability of a person holding an energized object to

control his muscles and release it.

3 9 mA to 25 mA This currents range may be painful and can make it difficult or

impossible to release energized objects grasped by the hand.

4 25mA to 60mA This currents range could lead to muscular contractions which

can make breathing difficult. These effects are not permanent

and disappear when the current is interrupted, unless the

contraction is very severe and breathing is stopped for minutes

rather than seconds.

5 60mA to 100 mA This currents range may cause ventricular fibrillation, stoppage

of the heart, or inhibition of respiration might occur and cause

injury or death.

If shock currents can be kept below this value by a carefully designed earthing system by

reducing the shock voltage (shock voltage can be reduced by protective equipotential

bonding) and duration, injury or death may be avoided. The magnitude and duration of

the current conducted through a human body at 50 Hz should be less than the value that

can cause ventricular fibrillation of the heart.

16.4 Resistance of Human Body

For D.C and 50 Hz A.C currents, the human body can be approximated by a resistance.

The current path typically considered is from one hand to both feet, or from one foot to

the other one. The internal resistance of the body is approximately 300, whereas values

of body resistance including skin range from 500 to 3000.The value of resistance of a

human body (RB) from hand-to-feet and also from hand-to-hand, or from one foot to the

other foot is to be taken as 1000 as per IS 3043-2018.



Figure 45: Illustration of body resistance for different part.



ANNEXURE-I EARTHING TERMINOLOGY

(Ref IS 3043:2018 & IS 732:2019)

i. Arc-Suppression Coil (Peterson Coil)

An earthing reactor so designed that its reactance is such that the reactive current to earth

under fault conditions balances the capacitance current to earth flowing from the lines so

that the earth current at the fault is limited to practically zero.

ii. Bonding Conductor

A protective conductor providing equipotential bonding.

iii. Class 0 Equipment

Equipment in ‘which protection against electric shock relies upon basic insulation; this

implies that there are no means for the connection of accessible conductive parts, if any,

to the protective conductor in the fixed wiring of the installation, reliance in the event of

a failure of the basic insulation being placed upon the environment.

iv. Class I Equipment

Equipment in which protection against electric shock does not rely on basic insulation

only, but which, includes an additional safety, precaution in such a way that means are

provided for the connection of accessible conductive parts to the protective (earthing)

conductor in the fixed wiring of the installation in such a way that accessible conductive

parts may not become live in, the event of a failure of the basic insulation.

v. Class II Equipment

Equipment in which protection against electric shock does not rely on basic insulation

only but in which additional safety precautions such as double insulation or reinforced

insulation are provided, there being no provision for protective earthing or reliance upon

installation conditions.

vi. Class III Equipment

Equipment in which protection against electric shock relies on supply at safety extra-low

voltage (SELV) and in which voltages higher than those of SELV are not generated.

vii. Principal Characteristics of Equipment and Precautions for Safety

Table below gives the principal characteristics of equipment according to this

classification and indicates the precautions necessary for safety in the event of a failure

of the basic insulation.

Table 4: Equipment Characteristics and Safety Precautions

Class 0 Class I Class II Class III

Principal

characteristics of

the equipment

No. means for

protective

earthing

Protective

earthing means

are provided

Additional

insulation and

no means for

protective

earthing

Designed for

supply at safety

extra low voltage.

Precautions for

safety

Earth-free

environment

Connection to the

protective

earthing

None

necessary

Connection to

safety extra-low

voltage



viii. Dead

The term used to describe a device or circuit to indicate that a voltage is not applied.

ix. Double Insulation

Insulation comprising both basic and supplementary insulation.

x. Reference Earth

The conductive mass of the earth, whose electric potential at any point of this mass of

earth is taken as zero with reference to an earthing system of electrical power system or

electrical installations in a building.

xi. Earthing System

Arrangement of connections and devices necessary to earth equipment or a system

separately or jointly.

xii. Global Earthing System

Equipment earthing system created by the interconnection of local earthing system that

ensures, by the proximity of the earthing system, that there is no dangerous touch

voltage.

NOTES

1 Such system permits the devices of the earth fault current in a way that results in

a reduction of the earth potential rise at the local earthing system. Such a system

could be said to form a quasi-equipotential surface.

2 The existence of global earthing system may be determined by simple

measurement or calculation for such system. Typical examples of global earthing

systems are in city centre, urban or industrial areas with distributed low and high

voltage earthing.

xiii. Earth Electrode

A conductor or group of conductors in intimate contact with and providing an electrical

connection to earth.

xiv. Earth Grid

Earth electrode in the form of two over lapping groups of buried, parallel, horizontal

electrodes usually laid approximately at right angle to each other with the electrodes

bonded at each inter section. Earth grid provides common earth for electrical devices and

metallic structures.

xv. Earth Electrode Resistance

The resistance to earth of an earth electrode or earth grid.

xvi. Earth Fault Loop Impedance

The impedance of the earth fault current loop (phase-to-earth loop) starting and ending at

the point of earth fault.

xvii. Earth Leakage Current

A current which flows to earth or to extraneous conductive parts in a circuit which is

electrically sound.

NOTE — This current may have a capacitive component including that resulting from

the deliberate use of capacitors.



xviii. Earthed Concentric Wiring

A wiring system in which one or more insulated conductors are completely surrounded

throughout their length by a conductor, for example, a sheath which acts as a PEN

conductor.

xix. Earthing Conductor

A protective conductor connecting the main earthing terminal (or the equipotential

bonding conductor of an installation when there is no earth bus) to an earth electrode or

to other means of earthing.

xx. Electrically Independent Earth Electrodes

Earth electrodes located at such a distance from one another that the maximum current

likely to flow through one of them does not significantly affect the potential of the

other(s).

xxi. Equipotential Bonding

Electrical connection putting various exposed conductive parts and extraneous

conductive parts at a substantially equal potential.

NOTE — In a building installation, equipotential bonding conductors shall interconnect

the following conductive parts:

a. Protective conductor;

b. Earth continuity conductor; and

c. Risers of air-conditioning systems and heating systems, if any.

xxii. Exposed Conductive Part

A conductive part of equipment which can be touched and which is not alive part but

which may become live under fault conditions.

xxiii. Extraneous Conductive Part

A conductive part liable to transmit a potential including earth potential and not forming

part of the electrical installation.

xxiv. Final Circuit

A circuit connected directly to current-using equipment or to a socket outlet or socket

outlets or other outlet points for the connection of such equipment.

xxv. Functional Earthing

Connection to earth necessary for proper functioning of electrical equipment.

xxvi. Live Part

A conductor or conductive part intended to be energized in normal use including a

neutral conductor but, by convention, not a PEN conductor.

xxvii. Main Earthing Terminal

The terminal or bar (which is the equipotential bonding conductor) provided for the

connection of protective conductors and the conductors of functional earthing, if any, to

the means of earthing.



NOTE —The conductors of the functional earthing may be connected to main earthing

terminal (which is the equipotential bonding conductor) only if the same is

recommended by original electrical equipment manufacturer.

xxviii. Neutral Conductor

A conductor connected to the neutral point of a system and capable of contributing to the

transmission of electrical energy.

xxix. PEN Conductor

A conductor combining the functions of both protective conductor and neutral conductor.

xxx. Portable Equipment

Equipment which is moved while in operation or which can easily be moved from one

place to another while connected to the supply.

xxxi. Potential Gradient (At a Point)

The potential difference per unit length measured in the direction in which it is

maximum.

NOTES

1 When an electric force is due to potential difference, it is equal to the potential

gradient.

2 Potential gradient is expressed in volts per unit length.

xxxii. Earth Potential

Electric potential with respect to general mass of earth which occurs in, or on the surface

of the earth around an earth electrode when an electric current flows from the electrode to

earth.

xxxiii. Earth Potential Rise

Voltage between an earthing system and reference earth.

xxxiv. Hot Site

Substation where the rise of earth potential under maximum earth fault condition can

exceed the value either 430V or 650 V depending upon the fault clearance time.

xxxv. Transferred Potential

Potential rise of an earthing system caused by a current to earth transferred by means of

a connected conductor (for example a metallic cable sheath, PEN conductor, pipeline,

rail) into areas with low or no potential rise related to reference earth resulting in a

potential difference occurring between the conductor and its surroundings.

NOTES

1 The definition also applies where a conductor which is connected to reference

earth, leads into the area of the potential rise.

2 Transferred potential can result in electrocution path through the human body

other than the ‘touch voltage’ path that is hand to hand.

xxxvi. Protective Conductor

A conductor used as a measure of protection against electric shock and intended for

connecting any of the following parts:

a) Exposed conductive parts,



b) Extraneous conductive parts,

c) Main earthing terminal, and

d) Earthed point of the source or an artificial neutral.

xxxvii. Reinforced Insulation

Single insulation applied to live parts, which provides a degree of protection against

electric shock equivalent to double insulation under the conditions specified in the

relevant standard.

NOTE —The term ‘single insulation’ does not imply that the insulation has to be one

homogeneous piece. It may comprise several layers that cannot be tested singly

as supplementary or basic insulation.

xxxviii. Residual Current Device

A mechanical switching device or association of devices intended to cause the opening of

the contacts when the residual current attains a given value under specified conditions.

xxxix. Residual Operating Current

Residual current which causes the residual current device to operate under specified

conditions.

xl. Resistance Area (for an Earth Electrode only)

The surface area of earth (around an earth electrode) on which a significant voltage

gradient may exist.

xli. Simultaneously Accessible Parts

Conductors or conductive parts which can be touched simultaneously by a person or,

where applicable, by livestock.

NOTES

1 Simultaneously accessible parts may be:

a) live parts,

b) exposed conductive parts,

c) extraneous conductive parts,

d) protective conductors, and

e) Earth electrodes.

2 This term applies for livestock in locations specifically intended for these

animals.

xlii. Supplementary Insulation

Independent insulation applied in addition to basic insulation, in order to provide

protection against electric shock in the event of a failure of basic insulation.

xliii. Switchgear

An assembly of main and auxiliary switching apparatus for operation, regulation,

protection or other control of electrical installations.

xliv. Voltage, Nominal

Voltage by which an installation (or part of an installation) is designated.

xlv. Touch Voltage

Voltage between conductive parts when touched simultaneously that is the potential

difference between an earthed conductor part of equipment, (that is exposed conductive



part) which can be touched and which is not a live part but which may become live under

fault condition and a point on a conductive part (that is extraneous conductive part)liable

to transmit a potential including earth potential and not forming part of the electrical

installation or a point on earth’s surface separated by a distance equal to the maximum

normal reach (hand to hand or hand to foot) approximately one metre (see Fig. 25).

NOTE — The value of the effective touch voltage may be greatly influenced by the

impedance of the person in electrical contact with these conductive parts.

xlvi. Prospective Touch Voltage

Voltage between simultaneously accessible conductive parts (exposed conductive parts

when energised under fault condition and extraneous conductive parts or mass of earth)

when those conductive parts are not touched simultaneously by a person.

Step Voltage at an Earthed Structure

Touch Voltage at a Grounded Structure

Figure 46: Step and Touch Voltages

E Step

RK

RFRF IK

Potential rise aboveremote earth during

short circuit E Step

R1R2 R0

R0

R2

R1

RF

RFRK

IK

I

K

IK

I

2

RF

2

E Touch

RK

IKRF

Potential Rise AboveRemote Earth During

Short Circuit E Touch

R1 R0

R0

R1

R



xlvii. Step Voltage

The potential difference between two points on the earth’s surface, separated by distance

of one pace, that will be assumed to be 1 m in the direction of maximum potential

gradient see fig. 25.

xlviii. Equipotential Line or Contour

The locus of points having the same potential at a given time.

xlix. Mutual Resistance of Earthing Electrodes

Equal to the voltage change in one of them produced by a change of one ampere of direct

current in the other and is expressed in ohms.

l. Earth Grid

A system of earthing electrodes consisting of inter-connected connectors buried in the

earth to provide a common earth for electrical devices and metallic structures.

NOTE — The term ‘earth grid’ does not include ‘earth mat’.

li. Earth Mat

A earthing system formed by a grid of horizontally buried conductors and which serves

to dissipate the earth fault current to earth and also as an equipotential bonding conductor

system.



REFERENCES

1. IS 3043-2018: Code of Practice for Earthing (Second Revision).

2. IS 732-2019: Code of Practice for Electrical Wiring Installations (Fourth Revision)

3. IS 9409-1980: Classification of Electrical And Electronic Equipment with Regard to

Protection Against Electric Shock

4. Central Electricity Authority (Measures Relating to Safety and Electric Supply),

Regulations -2010.

5. IEEMA Journals August, 2020 and October, 2020.

6. “The Missing Link-Guide on Electrical Safety in LV Installation”.



NOTE



CAMTECH is continuing its efforts in the documentation and up-

gradation of information on maintenance practices of electrical

assets. Over the years a large number of publications on electrical

assets have been prepared in the form of handbooks, pockets books,

pamphlets & video films etc. These publications have been uploaded

on the internet as well as rail net.

For downloading these publications please do following:

1. On internet visit :www.rdso.indianrailways.gov.in

Go to Directorates CAMTECH Other important links

Publications for download Electrical Engineering

2. On Railnet visit RDSO website at 10.100.2.19

Go to Directorates CAMTECH Publications for download

Electrical Engineering

For any further information regarding publications please contact:

Dy. Director (Elect.) BSNL: 0751- 2470740 (O)

Rly. : 03747202

: 9752447030 (CUG)

SSE/Electrical : 9755549297 (CUG)

E-mail : [email protected]

Fax : 0751- 2470841

Write at : Dy. Director (Electrical)

Indian Railways

Centre for Advanced Maintenance

Technology,

In front of Hotel Adityaz,

Airport Road, Maharajpura,

Gwalior, Pin code – 474 005

http://www.rdso.indianrailways.gov.in/

earthing system & protection in low voltage …

Documents